Need a break from webmastering? How about a game of Sauerbraten? It’s an open source First Person Shooter game with graphics and gameplay along the lines of Quake. There is very little singleplayer functionality, but the various online (or LAN) multiplayer modes are a lot of fun.
One interesting part of the game is that the entire map geometry is rendered in real time, using fractions of cubes. This makes it really easy to create your own maps to play on.
Perhaps the greatest part of the open nature of the game is that it’s cross platform. It works on Windows, Mac OS X and Linux.
I know this is only narrowly on topic, but it’s a fun game that won’t take up too much of your time (unless you want it to). How many fun and high-quality open source games are there?