Previously referred to as the 'Doom 3 Engine', Tech 4 is id Software's current licensing platform. Powering games such as Doom 3, Quake 4 and Enemy Territory Quake Wars, Tech 4 revolutionized the use of realtime dynamic lighting as well as other technologies.
Clean code, extendable, cross-platform, unified lighting, decent tools, easy to mod, Blender <> idTech 4 art workflow. Good ol' engine with a lot of potential.