NitroTracker homepage with downloads, screenshots, docs
There's plenty of potential for other homebrews on the DS, too. Here's just a quick overview of some of the possibilities:
NDS Homebrews: DS homebrew ROMS and reviews community, with plenty of links and resources; a good starting point for learning about homebrews (and yes, as always, there are caveats when you're learning to do this!)
Drunken Coders, news and resources for homebrew development on DS, GBA, SNES, and even Wonder Swan. Helpful if you're learning about homebrews or developing your own.
Tobw's simple sampling keyboard: Not as advanced as this tracker, but a fun toy
DS Wiki: Comprehensive info for homebrew users and developers alike; a must-visit
Make Blog How-To on running homebrew games with PassMe
Unlike Sony, Nintendo doesn't appear to be quashing this sort of development on DS, thankfully. Now I just need to clear an evening for some tracking goodness -- and Metroid, too. (CDM Metroid Wi-Fi clan, anyone?).