Doomsday is a source port for the Doom series of computer games, along with Heretic and Hexen.

Colloquially know as jDoom (even though it does not use Java), Doomsday is considered one of the best implementations of OpenGL into a Doom source port. It uses GLBSP to parse maps into a format more easily rendered, and includes the ability to use models replacing the sprites plus high resolution textures if downloaded. The standard model pack can be downloaded for around 9 megabytes, but the high resolution textures total into several hundred.

Doomsday's gameplay is fairly faithful to the original games, and the inclusion of models and hi-res textures gives a more realistic, modern feel. Map editing for Doomsday is somewhat subpar compared to other ports, although superior to the original.

A major feature lacking in Doomsday is Boom support, and one must use many of the rough old Doom 2 tricks, especially for things like 3d floors. Doomsday does have a proprietary scripting language called DED, and a custom linedef action language called XG. XG, however, is not as easy to use a Boom's generalized action system, although it allows one greater power in controlling the game. Lastly, Doomsday uses a script system called InFine to create interludes between missions.

Despite its editing faults Doomsday is extremely visually impressive, blowing non OpenGL ports out of the water in that respect. If one owns a Doom based game it's a must download (after all, its free!).

For further information:
http://www.doomsdayhq.com (for information, model packs, hi-res texture packs, and the actual engine itself!)
http://download.com.com/3000-7563-855496.html?tag=lst-0-2 (Doom Shareware. If you haven't played Doom, you haven't lived! Download this and follow the instructions that come with Doomsday to set yourself up!)

(Doomsday runs only under Windows currently, although there are plans for a Linux version)