(From the Doomsday Engine Wiki)
The Doomsday Engine is an enhanced Doom source port for the Windows, Mac OS X, and Linux platforms. It is based on the source code of id Software's Doom and Raven Software's Heretic and Hexen.
The purpose of the project is to create versions of Doom, Heretic and Hexen that feel the same as the original games but are implemented using modern techniques such as 3D graphics and client/server networking. A lot of emphasis is placed on good-looking graphics.
Mac OS X Only
Mac OS X 10.3, OpenGL capable graphic accelerator with 64 Mb of VRAM (ATI Radeon 7500 or Nvidia GeForce 4 MX based cards), one or more IWAD files from a supported game (If you don't own any of the original games you can always use the shareware/demo versi