- Tremulous and OpenArena use ioquake3, don't have his own engine.
- Team Fortress use QuakeWorld
- Team Fortress Classic use Half-Life (GoldSrc) engine.
- Counterstrike use Half-Life (GoldSrc) engine.
- Portal use Source engine.
- Kingpin uses the Quake2 engine with few modifications

the "hot" engines today seems Source, idTech5, ioquake3 and whetever the hell is called the last Unreal engine.