Tank digging battle game

Open Source voxel shooter

Dhewm 3 is a Doom 3 GPL source port

Shoot and survive

Game engine for Return to Castle Wolfenstein

Epic story-driven FPS running on the Build Engine

Leading the way in newschool multiplayer Doom online

EDuke32 is an awesome, free homebrew game engine

A free first person shooter for Mac, PC and Linux

Source port for Build engine games

Free agile arena shooter

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

Free fast paced arena shooter

The Free and Fast Arena Shooter

Modern cross-platform port of the Descent engine

First-person shooter game

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

A Quake mod based on the retro shooter SPRAWL

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

A modernization effort of DOOM-3-BFG

Game engine for Classic Doom

Top-down shooter action RPG

FPS aim trainer

Open source first person shooter

Classic arena first person shooter

Open source first person shooter

Fast paced, first person strategy game

First-person shooter with cooperative in-game map editing

The orb ponderer's arena first-person shooter

a modern QuakeWorld client focused on competitive online play