Free/Open-source First-person Shooter Games

A first-person shooter (Commonly called FPS) is an action video game that involves "an avatar, one or more ranged weapons, and a varying number of enemies". FPSs render the game world from the visual perspective of the player character. FPS was one of the first genres to use key technologies such as 3D graphics, online play, and modding. Enhanced realism combined with graphic violence has also made FPS a common topic in ongoing controversies over video games.

I have collected some quality free/open-source first-person shooter games. From this list, I have only tried ActionCube, which I really had fun playing with.