TI’s LaunchPad boards have a history of being both low cost and fully featured. There’s a board for each of TI’s major processor lines, and all of them support the same “BoosterPack ...
The Texas Instruments MSP430 Launchpad is pretty popular in hacks, likely due to its low price. TI has recently released a new C2000 Launchpad device that offers more power and peripherals for $17.