The link to the code is here: http://code.google.com/p/opendrums/
You can check out the code if you like, I released it under an MIT license so you can really do whatever you want with it.
There are a couple issues still. Mainly is the audio latency can be annoying sometimes, as it gets to be a half second off. Since I don't know much about programming audio, I can't fix it too much, but I'm doing my research and will try to get this resolved as fast as I can.
I tweaked the old code because there was something making it so that if you hit two pads at once, only one of them plays. This is no longer the case, you can hit as many pads as you like and they will all play.
The code is done with SDL and OpenGL, so it should work just fine on Windows or Mac too, you just need to compile it because I haven't made a compiled version for either of those platforms.
UPDATE (01/20/09): It seems there has been some trouble getting opendrums to compile. On Ubuntu you need the following packages:
- build-essential - In order to compile C++
- libsdl1.2-dev - For SDL
- libsdl-mixer1.2-dev - For SDL_Mixer
- libgl1-mesa-dev - For OpenGL