No one seems to have mentioned the Minimus[1] as an arduino compatible[2] alternative -they we're costing us £3 and have a lot more GPIO pins to play with[3]. The new version is powered by a ATMEGA 32U2 so it's got some grunt (and lets you play USB too).

[1] http://minimususb.com/ [2] https://github.com/pbrook/minimus-arduino/ [3] https://github.com/pbrook/minimus-arduino/wiki#pinouts