The Digispark is a tiny, Arduino compatible development board using the ATMEL ATtiny85 microcontroller. It has its own USB bootloader and built in USB capability, allowing you to program it with the Arduino SDK and also use it as native USB device.
It is similar to the Arduino line, only cheaper, smaller, and a bit less powerful.
With the ability to use the familiar Arduino IDE, the Digispark is a great way to jump into electronics, or perfect for when an Arduino is too big or too much.
Features:
Support for the Arduino IDE 1.0+ (OSX/Win/Linux).
Power via USB or External Source - 5V or 7-16V (12v or less recommended, automatic selection).
On-board 500ma 5V Regulator.
Built-in USB (and serial debugging).
6 I/O Pins (2 are used for USB only if your program actively communicates over USB, otherwise you can use all 6 even if you are programming via USB).
8k Flash Memory (about 6k after bootloader).
I2C and SPI (vis USI).
PWM on 3 pins (more possible with Software PWM).
ADC on 4 pins.
Power LED and Test/Status LED
Size: 18.5mm x 26mm including USB connector, 18.5mm x 17mm excluding connector.