Manufactoria: http://pleasingfungus.com/Manufactoria/
velocity raptor: http://www.testtubegames.com/velocityraptor.html
ultimate tick-tac-toe: https://github.com/kennycason/ultimate_tictactoe
git branching: http://pcottle.github.io/learnGitBranching/
git commit guessing: http://guesshub.io/level/1