Announcing Adafruit Gemma – Miniature wearable electronic platform
adafruit.com
adafruit.com
This little guy is only about 1" (25mm) in diameter with an 8k processor and 3 usable pins.
On the other hand, it is programmable straight from microUSB which is very cool and makes it more accessible. (Looks like she did a software USB implementation, I don't think that MCU has USB support.)