A free first person shooter for Mac, PC and Linux

Shoot and survive

First-person shooter with cooperative in-game map editing

Free agile arena shooter

Free fast paced arena shooter

The Free and Fast Arena Shooter

Source port for Build engine games

Dhewm 3 is a Doom 3 GPL source port

Fast paced, first person strategy game

A Quake mod based on the retro shooter SPRAWL

First-person shooter game

A Zandronum 3.0 fork with improved netcode, configurable movement and many small tweaks

Open source first person shooter

Game engine for Classic Doom

Open Source voxel shooter

Yamagi Quake II is an alternative client for id Software's Quake II

Classic arena first person shooter

Open source first person shooter

A modernization effort of DOOM-3-BFG

Epic story-driven FPS running on the Build Engine

a modern QuakeWorld client focused on competitive online play

FPS aim trainer

Modern cross-platform port of the Descent engine

A modern cross‑platform port of “STAR WARS™: Dark Forces” with mod support

The orb ponderer's arena first-person shooter

EDuke32 is an awesome, free homebrew game engine

Leading the way in newschool multiplayer Doom online

ReC98 source port of the first Seihou Project game

Aliens vs Humans, First Person Shooter game with elements of Real Time Strategy

Game engine for Return to Castle Wolfenstein