1. Ångström Distribution (Most widely used and recommended)
Here, we will discuss about the Ångström Distribution on BeagleBoard since you can find very big development community.
Beagleboard is very small but powerful, with the nice integrated Anstrom distribution, you can turn this little board to a desktop like environment! the beginner guide "HowToGetAngStromRunning" and successfully make it works within 30 minutes! The good things are that all the drivers are pre-built, so whatever you plug a USB-tp-ethernet, or a USB mouse or keyboard, all works like a charm.
2. Android for BeagleBoard (Google Mobile System)
5. QNX Neutrino