I registered for Github a while back but only recently started using it. I'm still getting use to Git and I'm still quite new but I've been slowly adding the code for a little Lua based game engine. I guess it's similar to Love2d but in a way I prefer to program, better code reloading and I'll probably add some 3d stuff. My main sticking point is choosing a 3d file format.
This codebase is mainly for quickly prototyping games and it should run on everything. If that game's fast enough then that's cool. There's only a game loop at the moment.
This is a good getting started post: http://ryanlowdermilk.com/2012/01/the-quickest-and-easiest-getting-started-w-github-article-youll-ever-read/