I've just finished a simple multiplayer platformer, using LibGDX. It's all complete (including open-source assets) but has simple gameplay: players must collect the most coins while avoiding (or jumping on) the baddies. It's designed to be used by someone who doesn't want to write all the boilerplate code to get something up and running quickly, and can then add their own gameplay elements.
Full source is here: https://bitbucket.org/SteveSmith16384/s ... rc/master/
Ideally, I wish there were more open source games that were complete and fully functional (including good assets in place) but with simple gameplay, i.e. ready to be easily extended into something more by other people. Any suggestions?