Teensy 3.1 ARM cortex M4 dev board

The Teensy line of development boards seem like some pretty powerful and compact (read teensy) microcontroller boards.  The latest version, Teensy 3.1 uses a Freescale 32bit ARM Cortex M4.  With 262K of flash, a whopping 64K of RAM, and running at 72Mhz, the Teensy 3.1 seems to be on the higher end of the horsepower range.  At under $20, I wouldn't mind giving it a try.

Loading code into the Teensy is accomplished via a boot loader, much like Arduino.  So, no JTAG/Debugger required.  Also, the website mentions that the Teensy boards are "mostly" compatible with Arduino sketches with the Teensyduino plugin to the Arduino IDE.

There are several USB examples for implementing various USB interfaces.  Keyboard, mouse, serial and HID are available.

The teensy 3.1 is $19.80 and is available here, check out the other Teensy boards.
http://www.pjrc.com/teensy/index.html