Harry Godden's site
Marble Computing is a game about logic and computing, with an extreme restriction of using marbles.
It is availible to buy on Steam.
The games source code can be peeked at here: /git/?p=fishladder.git
And the engine code (codenamed Voyager): /git/?p=vg.git
The game and the engine's source code are written entirely in pure C, for simplicities sake.