The Teensy is a breadboard-friendly development board with loads of features in a, well, teensy package. The Teensy++ breaks out all of the IO available on the AT90USB1286 to breadboard-friendly 0.1" spaced headers so you can hook up a load of peripherals.
The Teensy++ comes pre-flashed with a bootloader so you can program it using the on-board USB connection: No external programmer needed! You can program the Teensy in your favorite IDE using C or you can install the Teensyduino add-on for the Arduino IDE and write Arduino sketches for Teensy!
Note: This does not come with a USB cable, please check below for an appropriate one.