Xonotic - Xonotic is a free (GPL), fast-paced first-person shooter that works on Windows, OS X and Linux.
Unity - The multiplatform game creation tools for everyone.
Quake - Quake is a series of first-person shooter video games released by id Software.
Godot Engine - Feature-packed 2D and 3D open source game engine.
Counter-Strike - Counter-Strike is a PC-exclusive FPS video game developed by Valve Corporation.
Blender - Blender is the open source, cross platform suite of tools for 3D creation.