Thom wrote in to share his impressive open source operating system*, Pyxis OS. Based around the Arduino hardware platform, it adds some pretty impressive features, such as the ability to run programs from an SD card, read/write to a FAT filesystem, and easily display graphics and GUI elements on a color LCD. It looks like it could be lots of fun to play with, and if you want to peer under the hood, a great way to learn how to build complex systems on tiny microcontrollers.
*To be clear, the operating system itself is open source, however the C-language compiler for it is proprietary. An assembler is provided for free.