The Teensy is a complete USB-based microcontoller development system, in a very small footprint. It is similar to Arduino boards in that it uses an ATMEGA Atmel AVR processor, however not compatible. Even so, there is a software add-on for the Arduino IDE. They claim the USB communication is faster than the Arduino, since the the later uses USB-to-serial communication. As far as I understand, that is also the case with the new Arduino Uno.
The Teensy boards are much smaller than the basic Arduino boards, but still features a mini USB port. It is also available with header pins to fit on a bread board. PJRC lists a number of fun projects using the Teensy.