Simply put, SpaceChem is one of my favorite all time games. It combines a programming field (something I like to do) with a fun way to solve puzzles, somehow involving Chemistry in the process. And it has a leaderboards/ rankings system, so you can compare with the best of the best. I've put nearly 300 hours in this game, and it continues to be one of the most unique experiences I've ever had. Here's just a page for my SpaceChem puzzles, ideas, and other random things.