Today's offering from the free and open source grab bag is Tremulous.
Tremulous is an award-winning multiplayer first-person shooter with gameplay inspired by Alien and Savage. Playing as either humans or bug-like aliens, you'll hunt down the opposing team while protecting your base.
Each team earns upgrades as rewards for kills. The aliens are able to evolve to more powerful forms while the humans gain new equipment from their arsenal. Furthermore, each team has builder classes which allow them to move their spawn point to a more protected area and build useful structures to hinder and harm the enemy.
Tremulous is a standalone game powered by the Quake 3 engine. It is completely free to download and play, and installers are available for Windows and Linux.
The development team is hard at work on Tremulous 1.2, which is "coming soon, but the current version is well worth the download and a lot of fun to play.