Shooters

GZDoom is a classic first-person shooter engine for all classic Id games. A free and open source game

Unvanquished is a first-person shooter that includes elements of real-time strategy. A free and open source game

Crispy Doom is a free and open source port of Doom with enhanced-resolution, limit-removing, based on Chocolate Doom

Wolfenstein: Blade of Agony is a free-to-play first-person shooter similar to Wolfenstein 3D, Medal of Honor, and Call of Duty, but with a more dynamic game in the spirit of Doom