The Doomsday Engine is an enhanced Doom source port, 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.

License: free

Additional System Requirements: FreeDoom or a commercial or shareware
Doom/Heretic/Hexen wadfile.
A graphics card with hardware accelerated OpenGL support.
SDL, SDL-mixer, and SDL-net (svga-lib for console).
Soundpacks from Sycraft.org (for background music) if your system does not support MIDI.
Also see the links and futhermore take a look at jDUI (UI enhancement)