Open Source voxel shooter

The orb ponderer's arena first-person shooter

The Free and Fast Arena Shooter

Free fast paced arena shooter

First-person shooter game

A Quake mod based on the retro shooter SPRAWL

Free agile arena shooter

Shoot and survive

A free first person shooter for Mac, PC and Linux

Game engine for Return to Castle Wolfenstein

FPS aim trainer

Epic story-driven FPS running on the Build Engine

Source port for Build engine games

Dhewm 3 is a Doom 3 GPL source port

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

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

Tank digging battle game

Fast paced, first person strategy game

Open source first person shooter

Open Source voxel shooter

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

Game engine for Classic Doom

Leading the way in newschool multiplayer Doom online

Modern cross-platform port of the Descent engine

A modernization effort of DOOM-3-BFG

ReC98 source port of the first Seihou Project game

Classic arena first person shooter

a modern QuakeWorld client focused on competitive online play

EDuke32 is an awesome, free homebrew game engine

First-person shooter with cooperative in-game map editing