Look into the TI MSP430. It was built with low-power in mind. There's even an Arduino IDE port for it called Energia if you're not comfortable in C/C++. Check out http://www.43oh.com for forums. Also, I put together an aggregated list of links to tutorials from a really nice blog http://goo.gl/IFsrJi . Also, I should mention, these boards are < $5 shipped from TI and includes 2 chips and a USB cable!