The Teensy development board is an Arduino cousin. It is based on the ATMEGA from Atmel, has a real small physical size (18x31mm) and development tools for Windows, Linux and OS X.
Pulling its power from the host’s USB, and being pre-programmed with a bootloader that makes it dead simple to upload software, the Teensy is a real versitile beginners board. Its physical size and software library makes it useful in many areas. One of my favorite features is that it can act as a USB HID, simulating mouse movements or keyboard presses with very little extra effort.
For larger projects, there is the Teensy++ with more FLASH, RAM and I/O. The downside is that it is slightly larger, measuring 18x51mm.