MSP430 Launchpad from Texas Instruments is one of the cheapest microcontroller development boards (4.30 $), but it was lacking a free IDE (I mean free as in freedom, not as in free beer). Now there is Energia, an Arduino-like IDE for MSP430 Launchpad, which makes it easy to get started, no matter the technical level of the user.
The MSP430 Launchpad is a very nice development board, coming with 2 MSP430 flash devices, mini USB cable and PCB connectors. Now, with this user friendly IDE, I think that MSP430 Launchpad is one of the best low cost microcontroller development boards.
You can check out Energia’s official page at http://energia.github.com/Energia/