Perfect for small, simple, sewable projects, the Arduino GEMMA is a collaboration between Adafruit and Arduino.cc. The board features an Attiny85, 8K of flash, and 5 I/O pins. It also has a USB bootloader, so it can be plugged it into any computer and reprogrammed via USB — just like an Arduino. (Note that this will use 2 of the 5 I/O pins.)
The entire board takes up very little space, at only 1.1″ in diameter and 0.28″ thick. It’s also very low power, drawing just 9 mA while running. It also has a reset button, an on-board green power LED and a red LED on pin one for flashing fun! The board can be powered via USB or using an external battery, and has an onboard3.3V power regulator with 150mA output capability.
You can buy the Arduino GEMMA from Adafruit for just $9.95.