Mobile Frame Zero is a worthy project, and an awesome looking game. Though it isn’t open source, it deserves mention here for being crowdfunded on a well run Kickstarter campaign which is, as previously conceded, is a respectable model for a for-profit game. Add to this that the game’s miniatures are player-built Lego robots, and you’ve got a flexible model that leads to happy players.
Give the Mechanized Artillery what they want! What they seem to currently want is a really cool flickr group for open posts of lego mecha from people all over.