Jamulator

Using player-piano-like grids, finger paint drums, then choose from over 100 other instruments to create musical arrangements in just minutes. You can zoom in to arrange your notes with precision, or just swipe and see what sounds come out. Quantization makes most of your casual swipes sound interesting and musical.

Once you have something you like, you can forward it to friends so they can add parts to the jam session. When you think it's finished, you can publish it for all app users to listen to.

There is also a sampler that lets you record audio, and then play notes of different pitch using the same player-piano-like grid. You can make your voice sound like a chipmunk, or like Darth Vader.

A very high quality musical instrument using general MIDI 2 standard voices.

New in 2.1.0: Live realtime jamming. There is still a little lag, but because of the metronome it feels like you are actually jamming. You are certainly interacting musically in close to realtime.