This is program inspired by the app SoundPrism. It does no have all the features of SoundPrism but it allows you hack the code and personalize it.
Soundprism is a software midi controller that simplifies playing chords. I tried it and in a few minutes I was making nice songs. The best part is that, using the iPad, you have the multitouch screen and it feels really natural playing that thing.
I decided making my own basic version using Processing.
My version is very basic and does not include any of the nice features available in the Pro version of SoundPrism. This is a screen shot: