http://www.youtube.com/watch?v=RYlRF0szSVM
Basically a really well polished homebrew GH clone for the DS. It has a lot of video game music in it for the songs so it's kinda neat that way.
The game comes with 65 songs (all in MIDI... but.. *shrug*) I had a lot of fun with it.
If you have a DS flashcard you can play this game by downloading it from videogameshero.com