What would essentially be my first electronics project, I chose to try and clone the monome 40h kit offered by monome.org. Reasons for choosing this: 1) it's very cool 2) you can't buy them unless your lucky 3) the arduino firmware is very similar to the atmel setup the 40h logic kit uses.
The shield is mainly a max7221 and shift register breakout, the shield plus the arduino is designed to replace the 40h logic kit. Differences in usb to serial hardware resulted in the Arduinome team writing a better firmware and software router. It now enjoys even a Wikipedia entry where you can find out more information: http://en.wikipedia.org/wiki/Arduinome
Originally laid out on a breadboard, using the ladyada boarduino as the mcu of choice the arduino monome was made functional enough to run basic monome patches and used the standard monome serial application for osc message routing.
You can see the 1st arduino monome shield I created next to the latest blue 'special edition' of which 128 were made and are in the process of being sent around the world.
Ultimately it is the hardware side of things that interests me the mode, so an initial pcb for an arduino shield was developed. Little has changed from this initial design except the change from a pull down to pull up connection on the resistor array.
You can download the following files if you wish to make your own copy of the arduino monome shield: