Arduino is an open-source physical computing platform based on a simple i/o board and a development environment that implements the/ 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). The open-source IDE can be for free (currently for Mac OS X, Windows, and Linux).
This is the new, smaller, Arduino Mini with ATmega168 (double memory size of the standard Arduino ATmega8). This device is quite a bit smaller and requires an external serial connection for programming. Board comes fully assembled and tested with ATMega168 pre-loaded with bootloader.