Telephone - Because it does so much more than simply make and receive calls, the phone part of the iPhone is the device's most easily overlooked feature. With many people abandoning their landlines for mobile phones, the aisle in Office Depot with nothing but bulky handset telephones looks like a relic from another century. I still have a landline in my house, but I've used it maybe five times in the past year and a half.
Calculator - Not only does the iPhone come with an extremely useful simple calculator, in landscape view it expands to a full-featured scientific calculator. And the App Store even has several graphing calculator apps, most of them costing $0.99, that can easily replace a TI-84 that costs far more. The only time I can see a use for a standalone calculator is for students at university -- examiners will inevitably protest against students using an iPhone for their exams, for some odd reason.
Personal Organizer (diary, address book, appointment scheduler) - Remember those electronic organisers people use to carry around with them five years ago? Those seem quaint enough, but go back even farther than that and you might recall people carrying around little black books with paper entries for appointments, addresses, phone numbers, to-do lists - all completely irrelevant now thanks to the iPhone's Calendar, Contacts, and Notes apps.
Calendar - Other than the novelty of having pictures of cute puppies, supermodels, or a Far Side comic looming over today's date, the traditional paper calendar doesn't even come close to being as useful as the iPhone's built-in Calendar app.
Wall Clock/Alarm Clock/Watch - The writing has been on the wall for all three of these devices since mobile phones started becoming popular. I haven't had a wall clock in my home in at least ten years, and haven't owned an alarm clock or watch since getting my first mobile nearly six years ago. Not only can the iPhone tell you the time and date without even having to unlock it, the Clock app can give you the time anywhere in the world, it also has a full-featured alarm clock that will never die on you if the house's electricity goes out overnight. As if that wasn't enough, it also has a stopwatch and a timer. Wall clocks and alarm clocks can't compete with those features, and the only advantage a watch has over the iPhone's clock is that you don't have to haul it out of your pocket to see what time it is.
Rolodex - Speaking of antiquated items -- has anyone actually used a paper rolodex in the past ten years? -- the iPhone's Contacts app handily replaces this.
Dictionary - I chuckle a little bit every time I pass the electronic dictionaries in an office supply store. Sure, I could spend $30 and up for a device nearly the size of a netbook that does maybe 1% of what a netbook does. Or I could buy an actual paper dictionary and haul that sucker around instead. Or I could bypass both of those options and download any of a number of dictionary apps from the iTunes Store. I use the app from Dictionary.com [iTunes link]; there are probably better apps out there, but the one from Dictionary.com has a definite price advantage -- it's free.
Voice Memo Recorder - Whether it's one of the old-school recorders that still uses tapes or one of the new models that has flash memory, no standalone voice memo recorder I know of enables you to e-mail your recordings to someone with the simplicity of the iPhone's built-in Voice Memos app. (Also works with iPod touch 2nd Gen and up with an external microphone.)
Pen/Paper - If you hate the iPhone's keyboard, this one might not sit well with you. Personally, I have no issues with it, especially in landscape view -- I can type about 30-35 words per minute on the iPhone's keyboard -- so the iPhone's Notes app initially replaced pen and paper for all my "I need to jot this down" needs, at least until I discovered WriteRoom [iTunes link]. I still carry a pen around with me, but only for two rare instances: when I need to sign something, or when I need to write something down for someone else.
Digital Photo Frame - Want to display a shuffled series of pictures of your wife and kids on your desk at work? Buy an iPhone dock and use the built-in Photo app instead. The iPhone's screen may not be as big as some of the digital photo frames out there, but it has the distinct advantage of being far less expensive and far more versatile.
USB Drive - This is another one that's a little less cut-and-dried. If you're one of those geeks who carries around bootable OS recovery discs or slimmed-down versions of Firefox with you everywhere on a flash drive, then the iPhone won't be a viable replacement for your needs. But if all you need is portable access to your files, the iPhone has one major advantage over a USB drive: not only can you carry files around, you can also view them. And if you're already paying for MobileMe, you can use the iPhone to access your iDisk from anywhere, giving you access to 20 GB or more of files while on the go.
If you hate MobileMe or just don't feel like paying for it, you can get Dropbox [iTunes link] instead -- it does pretty much the same thing as the iDisk app, with one advantage: a 2 GB account is free. The only time I've ever needed to use my USB drive since getting my iPhone has been when I needed to print something on a computer that didn't have access to my e-mail; in every other instance, I'd just e-mail the doc to myself and bypass the increasingly irrelevant 512 MB flash drive I bought several years back when 512 MB seemed like a lot of space.
Wireless Mouse - There's a number of apps in the App Store that replicate the functionality of a wireless mouse, and all of them are less expensive than even the cheapest mouse you'll find. I use Air Mouse Pro [iTunes link], and since buying it, my old wireless
Mighty Apple Mouse has pretty much just gathered dust.
Remote Control - Those little "presenter" remotes that sell for $15 and up? Don't give them a second glance. Once again, Air Mouse Pro can replace all of the functionality of a "presenter" remote, and other apps, like Apple's own Remote.app [iTunes link] or VLC Remote [iTunes link] each give you access and control over your computer's media files that rivals or surpasses the abilities of those $100 media center remotes from Logitech. The white remote that came with my wife's MacBook (back when they were still free -- those were the days) has sat forgotten and forlorn on a shelf since I got an iPhone.
3G Modem/Dongle - Unfortunately, U.S. readers need not apply (thanks a million, AT&T!). For almost everyone else in the world, using your iPhone to tether over 3G completely obviates the need for a separate 3G modem for your computer, especially since iPhone data rates are often cheaper than the plans offered with standalone modems.
Maps/Atlas - I have a glovebox full of maps that I never use anymore thanks to the iPhone's built-in Maps app. Not only can you do away with using maps for directions, if you use Google Earth [iTunes link], you can pretty much do away with using an atlas as well.
Compass (iPhone 3GS only) - Thanks to the 3GS's built-in magnetometer, there's no need to carry around a separate compass for navigation anymore. And apps that take full advantage of the compass, like MotionX GPS [iTunes link] make hiking trails far easier than a traditional compass.
GPS Nav Unit - If you have someone in the passenger seat to navigate and call out directions for you, you can easily get by using the free, built-in Maps app. If you're on your own, there's a glut of turn-by-turn apps in the App Store that replicate the functionality of standalone units like a TomTom, and all of them cost far less.
Digital Camera (iPhone 3GS only) - This one will likely be a bit contentious, but there's a well-worn saying even professional photographers live by: the best camera is the one you have with you. That said, I wouldn't recommend the awful camera in any iPhone before the 3GS as a replacement for even the cheapest point-and-shoot camera. The low resolution and lack of adjustable focus on the original iPhone and the iPhone 3G make their cameras next to useless for anything but the most utilitarian of shots.
The 3GS's camera, with its slightly beefier resolution, the ability to adjust focus, and the capacity to shoot, edit, and e-mail video, all make its camera perfectly acceptable for day-to-day use. Would you want to shoot your vacation pictures with it? Probably not. Would you want to try and take artistic shots with it? Almost definitely not (although some people are advocating it). But if all you want is a camera you can take with you everywhere for the odd shot here and there, the 3GS's camera is a perfectly acceptable substitute for a cheap- to mid-range point-and-shoot.
Digital Audio Player - Steve Jobs called the iPhone the best iPod they've ever made, and he's definitely right about that. Once I got an iPhone, my old iPod 5G got sold off almost immediately. The only advantage an iPod nano or shuffle offers over an iPhone is smaller size and comparative indestructibility, making them more useful for workouts than the iPhone. In almost every other way, the iPhone is the king of the hill when it comes to music players.
Portable DVD Player - This will be the first of a few entries where if screen size is a major issue for you, you might want to argue the point. Portable DVD players generally don't have very big screens in the first place, though, and every one of them is extraordinarily bulky next to the iPhone. Rather than carry one of these monsters and a booklet full of DVDs around, just rip some DVDs using Handbrake and put them on your iPhone instead. (Note: legality of ripping DVDs varies depending on where you live and who you ask. Personally, I don't worry about it at all when it comes to DVDs I own, but I'm just a rebel like that.)
Amazon Kindle - Once Amazon released their free Kindle for iPhone app [iTunes link], I knew I would never, ever drop northwards of $250 on a Kindle device. Some people swear by the Kindle's ease of use and readability, and those same people would probably balk at reading an entire novel on the iPhone's much smaller screen. Plus, last time I checked you still can't buy Kindle books directly from the app itself. However, I'm willing to accept those tradeoffs if it means saving $250 and not having to carry around a separate device.
The Complete Works of Shakespeare - To some people it might approach sacrilege (or induce blindness) to read Shakespeare's works on the iPhone. To other people (like me) the ability to carry around every extant Shakespearean play and poem with me everywhere I go and read them anywhere, at any time, with the Shakespeare app [iTunes link] far outweighs those considerations. To haul around a ten-pound book or a 65 MB application that lets me easily search through the text: that is the question, and it's an easy one for me to answer.
Any classical novel with an expired copyright - With the Stanza app [iTunes link] and its access to Project Gutenberg, thousands of classical novels are at your fingertips. Why spend $15 on Frankenstein at Barnes and Noble when you can get it for free? Project Gutenberg saved me literally hundreds of dollars during my undergraduate coursework, and thanks to Stanza, I have free access to an entire library worth of books wherever I go.
Portable Game Player - If you want to play AAA titles like The Legend of Zelda: Phantom Hourglass on the DS or Crisis Core: Final Fantasy VII for the PSP, the iPhone is obviously not going to be your platform of choice. Twitch-heavy first-person-shooters also often don't translate well to the iPhone's touchscreen. But for casual games that you can just pick up and play in the supermarket line or on your lunch break, even Nintendo's DS can't compete with the vast sea of games offered in the App Store. Granted, the vast majority of those games are shovelware, but there are diamonds in the rough: iShoot [iTunes link], Flight Control [iTunes link], Hero of Sparta [iTunes link], Peggle [iTunes link], and 2079 [iTunes link] are just a few of the games that have gotten me through many minutes spent in queues or waiting rooms.
Pedometer - With built-in support for Nike+, a traditional pedometer simply can't compete with the 3GS or iPod Touch's ability to keep track of your workouts. If you don't want to spend $20 on the Nike+ shoe transmitter (contrary to popular belief, you don't need to use Nike's Nike+ shoes at all), or if you have an iPhone 3G with no Nike+ support, you can buy an app like RunKeeper [iTunes link] instead.
Flashlight - This is another example of "The best [thing] is the one you have with you." The iPhone's screen isn't as bright as the eye-piercing halogen monsters or the tiny but laser-like LED flashlights you'll find at the store, and it also doesn't double as a shillelagh the way a Maglite does. But with an app like Flashlight [iTunes link], you can easily navigate your way through the house if the power goes out or even grab someone's attention with a strobe effect in an emergency.
White Noise Generator - Tinnitus sucks. Trying to sleep in an otherwise silent room with a constant eeeeeeeeee piercing through your brain is pretty much the worst thing ever. You can either drop $50 or more on a decent white noise generator, or you can spend $0.99 on Ambiance [iTunes link] instead, with access to over 500 free sounds to help you relax and fall asleep.
Guitar Tuner - Other than picks, guitar tuners are probably the one thing guitarists lose most often. Fortunately, there's an alernative to going to the music store and buying a new one -- go to the App Store and get a tuner on your iPhone instead. There are many free ones, though paid apps GuitarToolkit [iTunes link] and Cleartune [iTunes link] appear to be among the best-rated and most feature-rich. (Also works with iPod Touch 2nd Gen and up with an external microphone.)
One More Thing:
Netbook - When people wonder why Apple hasn't gotten involved in the current netbook craze, there's usually three schools of thought on the subject. The first is that Apple simply doesn't care about netbooks at all and doesn't want to cater to that market. Second is that the perennially forthcoming Apple Tablet, if/when it comes out, will do to the netbook market what the iPhone did for cellphones and the iPod did for audio players. Third is that Apple already has a netbook equivalent on the market: the iPhone.
Whether you see the iPhone as an acceptable replacement for a netbook will depend on a few things.
First is your relationship with the iPhone's keyboard. As I said earlier, I have no problems with it at all, especially since OS 3.0 came out and extended landscape functionality to almost every app. Some people out there have freakishly huge hands compared to mine, however, and for them typing more than a few sentences on the iPhone's keyboard is tantamount to Purgatory. As for me, I've written entire posts for TUAW on the iPhone's keyboard without issue, including this rather lengthy one.
Second is how willing you are to spend a large portion of your day staring at the iPhone's small screen. 480 x 320 as a resolution isn't competitive with even the cheapest of netbooks, but when using Safari this issue is largely alleviated by the ability to zoom in and out using Multi-Touch.
Third is what you're going to be using the iPhone for. If you're a weekend business road warrior who needs to edit Excel spreadsheets on the fly, then maybe you should stick with an actual computer. If all you're going to be doing is browsing the internet, sending short e-mails, or composing short documents, so long as the keyboard and screen aren't a problem for you, the iPhone is a perfectly acceptable replacement for a netbook. It also has the singular advantage of running (an admittedly very slimmed-down version of) OS X without resorting to hacks.
Anecdote time: On a trip to the States, Canada, and Fiji back in July, I left my MacBook Pro at home in New Zealand and used my iPhone as my primary computer for over three weeks. I certainly wasn't able to edit or even view any of the photos or video footage I shot, but as long as I had access to Wi-Fi, I was able to do almost everything else generally reserved for my MacBook Pro. I kept on top of e-mail from friends and relatives, kept an eye on the weather and news, listened to music, wrote up the first draft of two 1000-word+ posts for TUAW, wrote up more than 2500 words of impressions from the trip, kept up with schoolwork by reading three Shakespearean plays, kept myself entertained on 12-hour plane rides across the Pacific and hours upon hours of waiting in airport terminals by playing Peggle and iShoot... and all with a device that, unlike any netbook out there, fits in my pants pocket.
Does the iPhone do everything a netbook can do? No. But does any netbook do everything the iPhone can do, as well as the iPhone can do it? Definitely not.
That's all for the anti-buyer's guide. There's probably at least one item on this list that you can point to and say, "Yeah, I already have an app for that. Please, Grandma, don't get me this for Christmas this year, or at least make sure you get a gift receipt if you do."
More than a few of the items on the list are certainly debatable, especially the last one, but the point remains: in only three years, the iPhone has seen an explosion in functionality that very few people could have predicted when it was first introduced. It's become the electronic equivalent of a Swiss Army Knife, capable of duplicating or outright replacing many standalone devices and items and rendering others completely obsolete. It seems somewhat counterintuitive that something that costs as much as an iPhone could actually end up saving you money in the long run, but if you can actually manage to replace all 33 items on this list with an iPhone as I have, you'll end up coming out way ahead.