Freedoom
- Volunteer project to create a free version of Doom data files (WADs). These can be used with any Doom engine. (3-clause BSD)
- http://freedoom.sourceforge.net/

Risen3D
- A Doom engine port based on Doomsday Engine. OpenGL rendering. (GPLv2) [Windows]
- http://r3d.doomwadstation.com/

Eternity Engine
- A Doom engine port based on Smack My Marine Up (SMMU). Includes scripting, portals, polyobjects, and Heretic support, and will power the upcoming Millennium and Mordeth TC modifications. Software rendering. (GPL) [Windows]
- http://doomworld.com/eternity/

Doom Legacy Wiki
- Community resource, with news, documentation and editing tutorials.
- http://legacywiki.net/

ZDoom
- Enhanced port for Windows. With compiled and source downloads, development news, tutorials for the new features and a discussion forum.
- http://www.zdoom.org/

Vavoom
- Source port that supports Doom, Heretic, Hexen and Strife. Screenshots, information, download, forums and editing information.
- http://www.vavoom-engine.com/

psDooM
- A process monitor and manager for *nix systems, using Doom as an interface.
- http://psdoom.sourceforge.net/

What is MBF, anyway?
- Brief information about the Marine's Best Friend Doom engine, for DOS.
- http://s91291220.onlinehome.us/doom/mbf.htm

ZDoomGL
- OpenGL port for Windows. Binary and source code downloads, and a discussion forum.
- http://zdoomgl.mancubus.net/

Client/Server Doom
- Port for deathmatch play on the Internet. Offers downloads for Windows, FreeBSD and Linux, and a discussion forum. No longer maintained.
- http://csdoom.sourceforge.net/

Doomsday HQ
- Home of the Doomsday Engine, jDoom, jHeretic and jHexen.
- http://www.doomsdayhq.com/

DOOM 7650
- Port of Doom to a Nokia cellphone
- http://www.wildpalm.co.uk/Doom7650.html

Legacy
- Engine with new features for enhanced deathmatch games, available for most major platforms. Contains discussion forums, development news, documentation, and links to derived engines.
- http://legacy.newdoom.com/

PrBoom
- A port for Windows and Linux that closely emulates the original game.
- http://prboom.sourceforge.net/

Doom: System Administration Tool
- Perform Unix administrator tasks with a Doom interface.
- http://www.cs.unm.edu/~dlchao/flake/doom/

PDoom
- DOS port, no longer updated. Source and binaries available.
- http://www.aurigae.demon.co.uk/doom.html

NTDoom
- Port to Windows NT, no longer developed. Binary and source downloads.
- http://www.s2.org/ntdoom/

Doom3D
- Windows port with Direct3D and splitscreen capabilities. Also supports full 3D stereoscopic effect.
- http://www.doomworld.com/doom3d/

WinDoom
- Windows port with higher resolutions and a console. Program download, source code, documentation and a front end available. Not updated.
- http://www.doomworld.com/doom4win/

Boom
- DOS port with editing features and a number of bug fixes. Compiled and source downloads and a feature list available.
- http://www.teamtnt.com/boompubl/boom2.htm