SquareWear 2.0 is an open-source, wearable Arduino microcontroller board. It uses Arduino (ATmega328) running at 3.3V and 12MHz as main processor. It has built-in mini-USB port for uploading programs, charging lithium batteries, and creating a serial communication channel. It comes with a lot of useful built-in components, such as a color LED, a general-purpose push-button, a buzzer, light sensor, temperature sensor, three MOSFETs (to drive high-current load).
It also has a built-in rechargeable lithium coin battery, so you can power your project right away without requiring external power supply. Every time you plug in the mini-USB cable, it charges the coin battery automatically. Better still, if you want a beefier battery, you can plug in an external lithium battery through the on-board battery jack. The built-in lithium charger can charge external battery as well. Overall SquareWear 2.0 packs a lot of useful features on a 1.7" x 1.7" board.