Mainboard is an S3 REST service clone, using MongoDB/GridFS as data storage.
It's based on the Padrino Framework (which in turn is based on Sinatra) and requires ruby 1.9. It has been heavily influenced by _why's Park Place, and Boardwalk.
Mainboard is a rack 1.3 compliant application, but so far only supports asynchronous servers (ie: thin). It has provisions for running in Cloudforge VCAP, taking autoconfiguration from the environment.