DarkPlaces

Darkplaces is the most advanced of the free software Quake engines. It has historically innovated most of the standard features which are later incorporated into other Quake engines. It also features an enhanced network code, with asychronous delta compression, and a built in server browser. It also features real time lighting and bump mapping. Darkplaces is actively developed by LordHavoc and contributors.

Nexuiz, Good Vs. Bad 2, Open Quartz and Transfusion are games using Darkplaces.

Zymotic, a game in development by the same team as Nexuiz, also uses Darkplaces.