Just when you thought the Arduino name permutations would stop – they don’t!
Enter the “Ardrumo”:

This is a software MIDI interface designed to accept serial data from an Arduino board and convert it into MIDI drum data in OS X. The application acts as a bridge between sensors connected to the Arduino board and applications such as GarageBand or Logic without requiring additional MIDI hardware. All data is transferred directly over the USB connection and no additional power source is needed.

Cool to see a project so fleshed out with interface software and documentation. Hit the link below for detailed info regarding piezo pad assembly as well.

Ardumo on Google Code -Link

Related:

Spooky Arduino Projects #4, and Musical Arduino -Link