Both the Arduino and the Raspberry Pi (pictured above) are stand-alone computing devices, have an array of expansion boards available, open source software, and either one can be purchased for about $30. However, the Arduino is a microcontroller, while the the Raspberry Pi is a full, modern computer– not as powerful as a laptop, but in some ways similar to a first generation Xbox.
Raspberry Pi homepage
Raspberry Pi wiki
Raspberry Pi expansion boards
