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.

GoQuake says

10/10 - Agree (2) Disagree (1)

It's free, runs well on older computers, allows you to legally shoot other people, and supports Windows, Linux and Mac OS X. What more could you ask for?