An Arduino set up with the monome serial protocol, running with MonomeSerial and several of the MaxMSP patches from the monome base collection.
As always, apologies for the video quality: hopefully you can occasionally make out what's happening on screen!
Read all about it at upnotnorth.blogspot.com/
2010 update: that Blogspot page is long abandoned. For the latest on this and (mostly) other projects, check out my new site at upnotnorth.net.