Open source first person shooter

Epic story-driven FPS running on the Build Engine

Classic arena first person shooter

Free agile arena shooter

Free fast paced arena shooter

Top-down shooter action RPG

Open Source voxel shooter

Tank digging battle game

Game engine for Return to Castle Wolfenstein

The Free and Fast Arena Shooter

Leading the way in newschool multiplayer Doom online

The orb ponderer's arena first-person shooter

Modern cross-platform port of the Descent engine

Open source first person shooter

Dhewm 3 is a Doom 3 GPL source port

Fast paced, first person strategy game

Open Source voxel shooter

Game engine for Classic Doom

A Quake mod based on the retro shooter SPRAWL

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

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

First-person shooter with cooperative in-game map editing

Source port for Build engine games

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

EDuke32 is an awesome, free homebrew game engine

Shoot and survive

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

FPS aim trainer

A modernization effort of DOOM-3-BFG

a modern QuakeWorld client focused on competitive online play