SpaceChem is ♥♥♥♥♥♥♥ brilliant. In the course of the game you use reactors to combine elements into molecules - that's pretty much the gist of it. However, it's done via timing mechanisms where you take in molecules and atoms and manipulate them via a series of steps - adding/removing bonds, fusing them, splitting them, teleporting them, the list just goes on. From an actual chemistry standpoint it doesn't make a hell of a lot of sense, but what's really intriguing is that this takes the essence of programming/circuit design and distills it into a fascinating game. Engineering students/programming students should CONSUME this game - it's a wonderful example of the sort of work we try to solve.
I got hopelessly addicted to this game for a good long while. It gets hard - REALLY hard. But the feeling of satisfaction upon finding a solution is just....so fantastic. Even more if you've got some friends who you can compete against trying to find more and more efficient solutions.
I cannot recommend this game enough. It's wonderful.