[Dimitri Diakopoulos] dropped into our tip line to let us know his HIDUINO project. The HIDUINO is a set of firmware for the ATmega8u2 used in the new revisions of the Arduino (Uno, and Mega2560 for example). Once the HIDUINO is loaded your Arduino can show up as any HID compliant device you wish, no extra drivers necessary. This means that using this firmware, music software such as Max, Ableton, Reaktor etc can read and write MIDI directly to/from the Arduino. Currently the project is for direct USB-MIDI communication but could be altered to act as a variety of HID devices.
Flashing the ATmega8u2 with your own device type or name requires a bit of work on the developers part like owning an ISP programmer, soldering header pins to the board, and re-flashing the ATmega8u2 every time you want to load new code into the Arduino. Users without an ISP can still flash HIDUINO using Atmel’s FLIP software (Windows) or the DFU programmer (Mac OS X and Linux) and a precompiled HIDUINO firmware. They still have to solder a jumper on the underside of the board to use the DFU bootloader.
A guide for ISP flashing can be found at [Dimitri]‘s site.