DarkPlaces is a game engine, used by Transfusion and Blood: Infuscomus, which is a heavily modified version of the Quake engine by id Software. Transfusion can also be run with other Quake engines, however it is primarily geared towards DarkPlaces. DarkPlaces is used in other major free software projects such as the original Nexuiz, its later fork Xonotic, and the free/commercial Steel Storm. It was programmed by Ashley Hale (LadyHavoc, formerly LordHavoc) and Transfusion is free software under the GNU General Public License.
As it is based on Quake, it is also largely compatible with the later id Tech engines. As such some maps from Transfusion have been converted to id tech 3 (Quake III engine) for Q3 BloodBath, while the mooted single player mode for Tranfusion was originally intended to utilize the Quake II engine under the title Caleb's Awakening. There is also a Caleb character model and bot available for Quake III (OpenArena compatible), made by Philip Stavitsky, as well as a Caleb player model for Kingpin: Life of Crime. Finally, there is also a fan fiction story by Kain-Xavier called "More Blood in the Arena Eternal" which creates a crossover between the storyline of Quake III and Blood II.