Do note that this is six addons I couldn't play without. Your opinions may differ; these are mine. None of these are for specific gameplay situations so much as more just for general quality of life in WoW. I'd love to see their features included in the standard UI by Blizzard, but until then, I'll carry on using them. You could give them a try too if you felt like it!
Addon Control Panel
Download Addon Control Panel: WoWInterface, WoWAce, Curse
This nifty addon is top of the list. Why is it top of the list? Because as the name suggests, it's an addon that allows you to control your other addons! It's all very meta. Seriously, though, if you're into addons like me, this is the one addon you should never be without. It allows you, by way of a simple screen, to switch on and off addons and modules of addons at will, without logging out of the game. I honestly don't know why this hasn't been included in the Blizzard interface as standard yet. Addon Control Panel proves it's possible, and it is just such a huge quality of life improvement for anyone who uses addons -- and that's most people.
You may also see that on the Altoholic section in the screenshot above, the modules of Altoholic are all loadable on demand. That means that all you need to do is click that Load button, and they're on and running. Pretty cool, right? No need to log out and back in again; just click a button.
Then, looking here at aIDamageMeter, that one doesn't have a Load button next to it. Don't worry! All you have to do is check the box to the left of the title, then reload your UI. Addon Control Panel even provides a handy Reload button to help you with that. About to zone into a raid and realize you've forgotten to switch on DBM? No problem. No need to log out; just check a box and reload your UI.
Download EasyMail: Curse
EasyMail, as the name suggests, is an easy addon that helps you manage your mail. How does it help you manage your mail? It works with two simple additions to the standard Blizzard mail UI. Firstly, it adds the ability to mark all your mail and get the attachments with two clicks. Rather than having to laboriously open every single message and take the attachments, it simply allows you to use two buttons, then sit back and wait a few moments.
Its other feature appears when you're trying to send mail. This program allows you to add (or not, as you prefer) all your guildies and friends to the mail auto-fill list. It remembers who you last sent mail to, who you've sent mail to and received mail from recently, and it auto-fills their name when you start typing. Again, so simple, but such a huge quality of life improvement!
Download Auctionator: Direct from Developer, WoWInterface, Curse
Auctionator simply improves the default Blizzard Auction House interface. Simply is the key word here, as you don't have to be a gold maker extraordinaire to benefit from this addon's incredibly straightforward features. I don't have the space in my word limit to go into great detail in examining all the excellent features of this addon, so I'm going to stick with the ones I use all the time.
Have a crafting profession? Need materials? Head to the Auction House, open your crafting pane on the item you want to craft, click an auctioneer, and then click the little AH button on your crafting pane. See how Auctionator searches and prices all the materials and the end product for you? Yeah. Pretty darn cool.
In terms of selling, there's a ton of useful features, but the one I use pretty much every single days is this: Go to the Auction House and click on an auctioneer. Open your bag. Hold alt, and click the item you want to sell. In one click, it puts it into the interface, searches other auctions for the lowest buyout price, and automatically undercuts it. Perfect!
Download Adibags: WoWAce, Curse
There are a lot of bag addons out there. Adibags is my favorite; your mileage may vary. What does it do? Well, first, it converts your separate bags into one big bag. It does the same for your bank. But it doesn't stop there! It automatically assigns categories to all your things, intelligently separating into character pane item sets, junk, trade goods, consumables, quest items, jewelry, armor, and weapons (the latter three only if they're not in a set) and empty space.
Download Chatter: WoWAce, Curse
Chatter, oddly enough, is a chat addon. No way! It's one I greatly fear may not be updated for Mists, because I love it. It is a simple yet ultra-customizable addon that allows you to set up your chat just how you want it. Again, I don't have the time to go into all its features here, but suffice it to say that it does simple things like allow you to automatically log chat, click links directly from chat, fully customize the look of your chat panels, color player names by class, invite players by clicking within chat, and more.
Download Bartender: WoWAce, WoWInterface, Curse
Last but absolutely, categorically not least: Bartender. The one addon that was most mentioned in the Breakfast Topic as being the one people couldn't survive without, and I have to say I agree. It's probably the one I've missed most in the beta. It simply makes your action bars completely customizable. It takes the maximum number of action bars provided by the standard UI and makes them moveable, customizable in shape, and reskinnable with a huge amount of addons that are designed to reskin Bartender. The sheer number of addon packs and UI replacements that include Bartender should be testament to just how fantastic this addon is. I even use it with TukUI. It's just so incredibly flexible and customizable, there simply isn't another addon that even comes close to Bartender for action bars.
Here are a couple of styling options with different skins, for your delectation.
And one of the very best things about Bartender is how easy it makes keybinding. No heading into the Blizzard menu and hunting down Button 9, ActionBar4. Simply type /kb in-game when you're out of combat, hover over the action bar button you want to bind, and press the key. Done! These binds can be saved by character (or not) and can be copied in two clicks with the use of profiles to new and old characters.
Addons are what we do on Addon Spotlight. If you're new to mods, Addons 101 will walk you through the basics; see what other players are doing at Reader UI of the Week. If there's a mod you think Addon Spotlight should take a look at, email firstname.lastname@example.org.