Arduino Pro Mini 328 - 5V/16MHz (Sparkfun - USA)
Description: It's blue! It's thin (0.8mm)! It's the Arduino Pro Mini! SparkFun's minimal design...
Hard to believe the power this little board has. You can prototype on a regular Arduino Uno and transfer it to the Pro Mini, but I bought a Pro Mini and attached header pins to it so I could prototype directly with it. Once everything works, it's a simple pin-for-pin transfer of the final design wires for your project.