Sparkfun just launched the Arduino Pro Mini for $19… it’s tiny, and cheap, and been in the works for a long time. Here’s their description:
It’s blue! It’s thin (0.8mm)! It’s the Arduino Pro Mini! SparkFun’s minimal design approach to Arduino. This is a 3.3V Arduino running the 8MHz bootloader (select ‘LilyPad’ within the Arduino software). The Pro Mini is very much like the Wee, but we added a handful of features to become a fully certified Arduino board.