Arduino compatible, Atmel SAM D21 chip, open source!
This is one of the smallest ARM powered boards in the world. If you are ready to transition away from AVR 8-bit hardware to the very powerful ARM 32-bit stuff, this is the way to learn! The board design, schematic, and parts lists are completely open-source.
FemtoUSB Board (Atmel ARM Cortex M0+) – [Link]