I have written about Arduino boards a lot, but there are also other small interesting quite similar development board recommended for me to check out. The Teensy is a complete USB-based microcontroller development system, in a very small footprint, capable of implementing many types of projects. All programming is done via the USB port and Teensy 2.0 boards are based on Atmel AVR micro-controllers, so they are quite similar to Arduino with USB with different form factor.
The main software development tools for writing software for Teensy is WinAVR C compiler. Most programs written for Arduino work on Teensy using Teensyduino add-on for Arduino IDE. All of the standard Arduino functions (digitalWrite, pinMode, analogRead, etc) all work on Teensy. Teensyduino is also compatible with many Arduino libraries.
There are also ARM based Teensy boards. Teensy 3.0 that costs $19. Teensy 3.0 is an affordable 32 bit ARM Cortex-M4 board, for development in Arduino or C/C++. RCArduino blog article Arduino Due has a nice compares several Arduino platforms and Teensy 3.0. New Teensy 3.1 has the same size, shape & pinout; is compatible with code written for Teensy 3.0, and is well supported on the Arduino IDE using Teensyduino.
There are several interesting Projects Using Teensy, and here is one video example of one: