Doom (Series)!

Xonotic is a free (GPL), fast-paced first-person shooter that works on Windows, OS X and Linux. The project is geared towards providing addictive arena shooter gameplay which is all spawned and driven by the community itself. Xonotic is a direct successor of Nexuiz with years of development between them, and it aims to become the best possible open-source FPS (first-person-shooter) of its kind.

Zandronum is a multiplayer oriented port, based off Skulltag, for Doom and Doom II by id Software.

Aion is a visually stunning MMORPG where your character wields devastating powers and sweeping wings to explore a celestial world.

Alien Swarm is a free top-down shoot-em-up video game by Valve Corporation. It is a remake of the Alien Swarm mod for Unreal Tournament 2004, and it was developed by the original team, which was hired by Valve during the course of the development process.

Shadow Warrior Classic Redux is a remastering of the cult classic PC shooter featuring enhanced visuals and new expansion content.

Freedoom is a free first-person shooter under continuous development. The Freedoom project aims to create a complete free content game based on the Doom engine. Combined with the engine, Freedoom is also compatible with game modifications made for the original Doom games, made by Doom fans and artists over the decades.

The Hammer of Thyrion project is a cross-platform source port effort of Raven Software's Hexen.

Alien Arena is a free and opensource first-person shooter game that combines a 1950s-era sci-fi atmosphere with gameplay similar to Quake (series) , Doom (Series) and Unreal Tournament (series) . Alien Arena focuses mainly on online multiplayer action, although it does contain single-player matches against bots.