Smoothieware Mini is a free, opensource firmware for an accessory system designed to run on ARM Cortex-M0 and run as extensions of a Smoothieboard, though they could just as easily be driven from another platform such as RAMPS or a Raspberry Pi. Eventually the system may also be ported to AVR.
The first prototype accessory boards have been made and the design of the core firmware system is well under way. Hopefully the first accessory boards will be available (complete with working firmware) in early 2013.
Nothing yet until things are further along :)