A recent side project of mine involved using Max and Ableton Live to build a patch that allows me to play my iPad like a cello. Rotating the iPad mimics the action of pulling a bow across the strings. When you change directions, it starts a new note. The volume and expression of each note changes in real time depending on how fast you rotate the iPad.
But the best part is that you don't need any musical knowledge to perform with this virtual cello, because CelloPad chooses the notes for you! I built a small probability distribution curve to compose random melodies within predetermined chord progressions. All you have to do is tap the screen to advance to the next chord, and CelloPad writes the melody with every new note.
Here's a demo video...with apologies to Radiohead. Remember, I'm not choosing any of the notes except when I play the root note of a chord. I'm mostly just using the iPad as the bow—the cello is playing itself!
Special thanks to Jane Arnison, Sjoerd van der Sanden, and the Max for Live community forum for their help with this project.