Free, open source shooter games

BZFlag
BZFlag (an abbreviation for Battle Zone capture the Flag) is a free, open source, online multiplayer video game.

Chromium B.S.U.
Chromium B.S.U. is an arcade-style, top-scrolling space shooter available on Windows, iPhone, PSP, Mac, Linux and numerous other UNIX-like operating systems.

CodeRED: Alien Arena
Alien Arena is a free, stand-alone first-person shooter computer game based on source code released by id Software.

Fore Thought Entertainment QuakeWorld
Fore Thought Entertainment QuakeWorld is one of the modern free software Quake engines.

Gang Garrison 2
Gang Garrison 2 is a freeware, open-source, indie video game "demake" of Valve Software's Team Fortress 2.

Nexuiz
Nexuiz is a free first-person shooter video game developed and published by Alientrap.

OpenArena
OpenArena is a free and open source first-person shooter computer game based heavily on the Quake III Arena style deathmatch.

Red Eclipse
Red Eclipse (often abbreviated RE) is a multi-platform, free, and open source first-person shooter that runs on top of the Cube Engine 2.

Tremulous
Tremulous is a free and open source team-based first-person shooter with real-time strategy elements.

World of Padman
World of Padman (WoP) is an open source first-person shooter computer game available in both English and German.

Xonotic
Xonotic is an open-source first-person shooter video game.