GOG.com, formerly Good Old Games, is a DRM-free digital video game distributor that originated with re-releasing old games, but has since expanded into new releases as well. Since August 26, 2015, GOG.com distributes games published by Bethesda Softworks, including id Software's titles.
Classic Doom games
- The Ultimate Doom ($5.99 / €5.39) 
- Doom II, the Master Levels, and Final Doom ($9.99 / €8.99) 
Doom engine games
Doom 3 engine games
How to use source ports with games downloaded from GOG.com
For other source ports, the following steps are necessary:
First, download the archive that contains the source port executable and other files necessary to run it. Then decompress the source port archive, and put all the files/folders into the game's installation directory. Running the source port main executable (for example, zdoom.exe or prboom.exe) should then start the game.
Alternatively, the IWAD (named doom2.wad for Doom II, hexen.wad for Hexen, etc.) can be copied from the installation directory to a directory of your choosing and the source port files decompressed/copied there. Again, running the source port main executable should then start the game.
- Final Doom uses the alternative id Anthology versions of the engine and IWAD files, rather than the more common original release.