Source Ports
Doom was open-sourced in the late 1990s, and many advanced derivatives have evolved from it. These are the source ports I use and/or recommend for playing my mods. There are many more out there that vary widely in purpose and capability.
GZDoom
GZDoom is an OpenGL source port based on ZDoom (and kept updated with it). In addition to the advanced editing features of ZDoom, it offers true colour support, dynamic lighting, skyboxes, and more. Like ZDoom, it supports nearly every Doom engine game, including Hexen and Strife.
GZDoom is my preferred source port for both playing Doom and developing mods. Unless otherwise noted, everything on this site is intended for GZDoom.
At the time of writing, the latest stable version of GZDoom is 2.1.1 (OpenGL 3.0+) / 1.9.1 (legacy OpenGL). Development builds are available from DRDTeam.
GZDoom is more demanding than some other source ports, especially with complex maps and high graphics settings, but should not be a challenge for modern computers. It will run on OS X and Linux, but you will have to build it yourself. It is open source, although the licensing situation is complicated. You can get the source from the Git repository.
ZDoom
ZDoom is an advanced source port, offering support for all Boom features and much more, including the UDMF map format and a heavily extended version of the ACS scripting language. The extensive feature set allows modders to go far beyond what Doom was originally capable of. It supports nearly all Doom engine games, including Hexen and Strife, and allows most originally game-specific features to be used across games.
Every source port on this page is based on ZDoom, and ZDoom is nearly identical to GZDoom apart from the renderer. Most mods on this site intended for GZDoom will run on ZDoom, though they probably won't look very good.
At the time of writing, the latest stable version of ZDoom is 2.8.1 (Windows | Linux x86 | Linux x64 | Mac). Development builds (Windows) are available from DRDTeam.
ZDoom is less demanding than GZDoom and will run on a greater variety of hardware. It is open source, although the licensing situation is complicated. You can get the source from the Git repository.
Zandronum
Zandronum is a client/server multiplayer source port based on GZDoom and directly descended from Skulltag. It offers most of the same features, along with several of its own multiplayer-specific enhancements. To the best of my knowledge, this is what everyone uses to play Doom online now.
Zandronum is based on an earlier version of GZDoom and has some differences, so not all mods will run. However, I have made a multiplayer map tweaked for Zandronum.
At the time or writing, the latest stable version of Zandronum is 2.1.2 and the latest development version is 3.0 (all versions). Development builds are released periodically on the forums and the source is available in a BitBucket repository.
GLOOME
OpenGL-Oriented Original Media Engine (conveniently contracted to GLOOME) is rebranded, GPL-compliant fork of GZDoom with a few additional developer-oriented features. The software renderer and a few other features have been removed to resolve licensing issues, and it uses OpenAL instead of FMOD for audio. GLOOME is intended to be used by developers to build original games rather than Doom mods, and was created in part for the Project 67 and Nocturne in Yellow indie game projects. However, it retains the ability to run Doom and many ZDoom mods.
GLOOME is based on the older 1.8 version of GZDoom and is deliberately not kept up to date with ZDoom/GZDoom, so mods meant for newer versions of GZDoom may not work. Conversely, it has some features GZDoom lacks so GLOOME games may not run on GZDoom.
GLOOME is intended for developers and is normally only available as source from GitHub, though occasionally a binary build will be released by the developers or the community. Most of the licensing mess has been resolved and GLOOME is licensed under a mix of GPL and GPL-compatible licenses.
GZDoom-GPL
GZDoom-GPL is a GPL-compliant fork of GZDoom maintained by Nash Muhandes and based on initial work by LavenderMoon. It differs from GLOOME in that it stays as close as possible to mainline GZDoom and is kept mostly up-to-date, only differing to the extent necessary to ensure GPL compliance. From a user's perspective, GZDoom-GPL is identical to GZDoom except that it is missing the FMod Ex audio backend, software renderer, and OPL synth.
GZDoom-GPL is only available as source from a repository on GitHub. It is intended solely for developers and they are expected to build their own binaries, possibly modified.
D-Touch
D-Touch is a Doom port for Android that actually includes three engines: GZDoom, PrBoom-Plus, and Chocolate Doom. As far as I know, the GZDoom part is based on GZDoom 1.8 but it might include a 2.x development build now. It will work with almost all GZDoom mods, including most of those on this site.
D-Touch used to be available on the Play store, but was pulled for unknown reasons. It's now available on the Amazon Appstore. The source code is available upon request as per the terms of the GPL.