M3 Adapter to get you started. I got mine from
Divineo, and they're now selling ones with a Passkey2 (which is required for running DS code through the GBA cart slot on a new firmware DS, a la Hot Rod Red). I don't have time to get links for all this stuff, but so far I've used Moonshell (MP3, OGG, etc. player), NDS2key (wireless keyboard emulator w/ wi-fi network sniffer), and DSLair (Dragon's Lair FMV port in development). There's a lot of other really cool stuff out there like NDS2Win, which is a wireless virtual mouse (more like a tablet or touchpad), that even displays screencaps of your PC desktop, SNESDS (SNES emulator), NESDS (NES emulator), SCUMMDS (Lucasarts adventure game engine emulator), Texas Instruments graphing calculator emulators, drawing apps, games, and much more out there.
Homebrew on the DS is awesome, and getting better. I just hope it won't get killed by the DS-Lite. I had to flash a new firmware to my DS to be able to DS stuff through my GBA slot without a Passkey, and they could easily remove the user flashability of the DS with a hardware revision, or at least make it much more difficult.