mbed is an ARM based fast prototyping board that uses an 100Mhz LPC1768 microprocessor from the Cortex M3 family. A small, breadboard friendly DIP package houses a wide range of peripherals, with programming, power and additional comms through a mini USB conector.
Interfaces supported include USB, SPI, I2C CAN, ethernet, and serial - all you need to hook up are some wires and the relevant connectors. Programs are written in C/C++ with an online compiler - this means whatever flavour of machine/os you use (Windows, Linux, Mac) you'll always have the latest version of the software on your desktop without any installation worries. To load the binary to the mbed itself, you simply save the file to the mbed's built in flash drive.
If you're not happy using an online compiler, there are ways of compiling code offline - here's one from dev.frozeneskimo.com. There are other methods available as well.
The mbed has a growing and supportive community - a wide range of libraries are already available supporting common hardware types like GPS or WiFi. There are already existing development boards that house the mbed. Additional hardware is in the pipeline to make the mbed experience even faster and even more convenient.