Adafruit GEMMA V2 – Miniature wearable electronic platform

Super small, only 1.1" / 28mm diameter and 0.28" / 7mm thick.
Easy-to-sew or solder pads for embedding in your wearable project
Low cost enough, you can use one for every weekend project
ATtiny85 on-board, 8K of flash, 512 byte of SRAM, 512 bytes of EEPROM
Internal oscillator runs at 8MHz
Ultra low power, draws only 9 mA while running
USB bootloader with a nice LED indicator looks just like a USBtinyISP so you can program it with the Arduino IDE (with a few simple config modifications)
Mini-USB jack for power and/or USB uploading, you can put it in a box or tape it up and use any USB cable for when you want to reprogram.
We really worked hard on the bootloader process to make it rugged and foolproof
~5.25K bytes available for use (2.75K taken for the bootloader)
On-board 3.3V power regulator with 150mA output capability and ultra-low dropout. Up to 16V input, reverse-polarity protection, thermal and current-limit protection.
Power with either USB or external output (such as a battery) - it'll automatically switch over
On-board green power LED and red pin #1 LED
Reset button for entering the bootloader or restarting the program.
3 GPIO - The 3 independent IO pins have 1 analog input and 2 PWM output as well.
Hardware I2C capability for breakout & sensor interfacing.