Hey, you could be like Space Engineers and be OSS even though they are selling it still ;)
https://github.com/KeenSoftwareHouse/SpaceEngineer...
Granted, I think they keep it a "Few" revisions behind just for the sake of still making money. Plus prototypes being open source doesn't mean that the full game has to! ;)
Note: OSS just means the source code is avalible, this does NOT mean that they are Free