Arduino is an open-source physical computing platform based on a simple i/o board and a development environment that implements the Processing/Wiringlanguage. 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 downloaded for free (currently for Mac OS X, Windows, and Linux).
This is the new Arduino Duemilanove. The Duemilanove automatically selects the appropriate power supply (USB or external power). A great description of the new functionality can be found here (USB fuse protection, software reset, and much more!). Board comes fully assembled and tested with ATMega328 pre-loaded with bootloader. Please see the Arduino support forum for more information about this new board revision and bootloader.