Cube2: Sauerbraten - Another excellent open source FPS. One of the coolest things about it is that you can build and edit levels while you're in the game. There's also Quadropolis, a massive repository of goodies for the Cube engine including maps, scripts, and mods.

Eat the Whistle - If you're into uber-realist sports games, go ahead and scroll right past this one. If, however, you enjoy a little bit of humor and some old-school visuals, Eat the Whistle is a must-have download. Features sim, career, and arcade (complete with power-ups) modes.

FreeCiv - Another cornerstone of open source gaming. Inspired by Sid Meier's classic series, FreeCiv challenges you to develop your civilization from primitive roots into a thriving, modern society. Multiplayer mode support up to 30 (yes, 30) players. With more than 100 playable nations and 29 translations, FreeCiv is a truly international Time Waster.

FreeDroid - A modern take on the C64 classic Paradroid. It's a rich, futuristic RPG with plenty of robot butt kicking. In addition to dozens of different weapons, there's also a wide range of magic attacks, and you can even take control of your enemies and use them against each other.

Frets on Fire - If you've been looking for an excuse to rock out with your keyboard, go download Frets on Fire now. Tell your office mates your learning to play like Vadim Pruzhanov. Sure, he plays a different kind of keyboard, but it's still badass.