LeafLabs Maple

So I'm a little late to the party on this one (read:  an entire year), but there's another open-source microcontroller available to the discerning DIYer that is compatible with all the Arduino add-ons (called "shields" in the 'biz).

The Maple by LeafLabs (licensed under Creative Commons) is distinct enough from the Arduino platform to deserve its own place in the microcontroller world.  Perhaps the first difference people will notice is that it's much, much, MUCH faster.  Tests by Make:Zine's staff put the update speed of the digital/analog converter, the component responsible for reading sensors and passing information to the processing unit, at 9x that of the Arduino Duemilanove.  This further opens the applications of the Maple to things like live audio editing, real-time GPS processing, and really anything that requires more bandwidth than you might want to squeeze out of your Arduino.