Open source first person shooter

Game engine for Classic Doom

Free agile arena shooter

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

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

Source port for Build engine games

FPS aim trainer

The Free and Fast Arena Shooter

Open Source voxel shooter

Free fast paced arena shooter

Game engine for Return to Castle Wolfenstein

EDuke32 is an awesome, free homebrew game engine

Fast paced, first person strategy game

Open Source voxel shooter

First-person shooter game

Fast arcade first person shooter

First-person shooter with cooperative in-game map editing

Classic arena first person shooter

Modern cross-platform port of the Descent engine

Open source first person shooter

A modernization effort of DOOM-3-BFG

The orb ponderer's arena first-person shooter

ReC98 source port of the first Seihou Project game

Tank digging battle game

Epic story-driven FPS running on the Build Engine

Leading the way in newschool multiplayer Doom online

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

Top-down shooter action RPG

Shoot and survive

a modern QuakeWorld client focused on competitive online play