In terms of DS-extending software, DSOrganize stands alone in its class. It plays music, it runs files, it (of course) functions as an organizer. It has a web browser and can download demos. It even has a voice recorder. But it's not the whole world of applications designed to make your DS do amazing things.
We've rounded up some apps that make your DS into more than a DS. Ever wanted to use your DS to find networks? Or to play PC games? Read on and find out how you can do those things, and more! Well, we'll just tell you right now how to do it, in general: by downloading stuff and running it on your DS. But read on for specifics.
DS2Win Fusion uses the DS's Wi-Fi capabilities to connect to your PC, at which point you can connect to your desktop remotely. The top screen shows your full desktop, and the bottom screen is a configurable zoomed view. You can do all your normal PC stuff, using the DS as your mouse and keyboard (and gamepad, and of course monitor).
DS2Key is a suite of two programs -- one for PC, and one for DS -- which turn your DS into a wireless PC controller. Like Joy2Key, which maps key presses to gamepad buttons, DS2Key allows you to set a DS button to a particular key. But DS2Key also allows you to use the DS "natively" as a gamepad -- as in, not a gamepad pretending to be a keyboard. In gamepad mode, you can use the touch screen as an analog joystick!
Cooking Timer is a simple program, but it really doesn't need to be any more complex: it plays the music file of your choice when a counter elapses. It's certainly cooler than any kitchen timer we've had. If you want the same thing as an alarm clock (and of course you do!), there's Morning Timer.
DS2DS allows two DS systems running the software to share files on the DS card. It's a simple idea, but quite useful. It's the promise of the Zune's social features, but not crippled. Or, to use a more Nintendo-appropriate analogy, it's like trading Pokémon, but interesting. It's originally in French, but an English translation patch here.
DSFTP turns your DS into an FTP server, allowing you to transfer files to and from your computer to your DS without removing your SD card. Of course, it also allows you to transfer files to and from other people's computers. Even more impressively, you can run things straight from the FTP prompt, so you don't even have to copy them! It's only officially designed for the SuperCard CF, but it seems to work well on other cards.
Sniff_Jazzbox isn't just a Wi-Fi network finder, though that in itself would be pretty cool and potentially useful. It's a bizarre musical instrument of sorts that translates the names of wireless networks it finds into tones. It gets more cacophonous (or melodious!) in the presence of more networks. Wardive, from the same developer, turns Wi-Fi signals into elements in an action game, in which you attempt to protect a crystal from the invading networks by tapping them.
SvSIP allows you to use your DS to communicate over voice-over-IP networks. You need an account with a voice-chat site that uses the open SIP protocol (the program's website has a list of such sites, including Freephonie and Wengo), but once you've overcome that minor hurdle, and configured the extremely fiddly connection settings, you can totally call people's phones. From your DS. May we suggest a headset?
Rickroll DS puts a powerful Rick Astley-based weapon in your pocket. Unable to Rickroll your friends at their computers? It can be pretty difficult, now that everybody expects a Rickroll. But on the DS, it's still novel -- still a surprise. Plus you can spring a Rickroll on someone at pretty much any time, even when there's no computer around. You'll be a Rickrolling machine!
Some of this stuff is actually useful, some is theoretically useful, and some is pretty much for novelty value only. But all of them make clever use of the DS hardware to extend its abilities beyond what would normally be expected of a handheld gaming system. And, of course, they may help defeat the argument that homebrew is just used for piracy.