Specifications:

Support for the Arduino IDE 1.0+ (OSX/Win/Linux)
Power via USB or External Source - 5v or 7-35v (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 (on Pin0)

Just how small is it? Well it's about the size of a quarter. While the small size made hand soldering prototypes difficult, we find that it is the perfect size to stick in even our smallest projects, but not too small to comfortably handle. Since it uses standard 0.1 in spaced headers it is easy to hookup to anything.

USB On-Board: With the on board USB connector it is also easy to program by wither directly plugging it into the computer or, if it is installed in a project, with a super cheap and common USB extension cable. In addition, it is small enough to be left plugged in your computer, use it to run a computer controlled RGB status LED, read a directly solder sensor, or fool your friends by programming it to be a USB keyboard and turn their caps lock on and off randomly, the possibilities are endless!