Atomix is a fun little game that falls in the category of "edutainment", or games which actually have some educational value. The object of the game is to move molecules around the playing area either vertically or horizontally until you can form an atom. The lower left corner of the screen displays the atom which you need to form, along with what the atom actually represents. Naturally the more complex the atoms are, the more complex the puzzles become. The only criticism that I have about Atomix is that there is a time limit which can't be turned off, but fortunately the game can be paused with the space bar. It's an enjoyable game if you have an interest in chemistry, and a great example of old skool fun!
Links: Atomix for Commodore 64, Atomix for PC.