Support for the 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)