Arduino is an open-source physical computing platform based on a simple i/o board and a user friendly development environment that implements the Processing/Wiring language.

Arduino can be used to develop stand-alone interactive objects or can be connected to software on your computer (e.g. Flash, Processing, MaxMSP, PD).

Board comes fully assembled and tested with ATMEGA2560 pre-loaded with bootloader.

Wikipedia: Arduino-Platform

Arduino Mega2560 Details

Arduino Homepage