OpenArena is a community-produced deathmatch FPS based on GPL idTech3 technology. There are many game types supported including Free For All, Capture The Flag, Domination, Overload, Harvester, and more. There are 17 unique player models to choose from and 12 weapons.
This game....just... Wow. It was extremly laggy, and I had to go to the setup screen and squint my way through the barely-readable text to lower the resolution and make it not fullscreen. But that didn't do anything at all. It was still laggy, and when I started the game, it crashed five seconds later.