If you like microcontrollers, you’re gonna love this…
Feel free to see this as blatant advertising, but you should really check out Texas Instrument's MSP430 LaunchPad. For the ridiciulously low price of $4.30 (shipping included) you get a complete microcontroller SDK with USB connectivity and JTAG debugging! The fully assembled circuit-board has generic I/O pins and peripherals; one switch and two LEDs. Two microcontrollers (one with I2C support!) and an USB cable are included in the package.
One of the best things about it is the software development enviroment; Code Composer Studio, which should reach version 5.1 by now. Built on vanilla Eclipse it allows for easy development and debugging. The sample projects are quite good at showing you how micro-controllers should be programmed if you are more familiar with CPU's like on desktops. A cool feature is called Grace which allows n00bs like me to set up a software design skeleton.
I almost forgot to mention it has complete Linux support. That is an excellent move on Texas Instruments's behalf. Mad props to them for bringing such a mutual beneficial product!