GOG.com

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, GOG.com distributes games published by Bethesda Softworks, including id Software's titles.

List of Doom games and other related games available on GOG.com

 * The Ultimate Doom ($5.99 / €5.39)
 * Doom II, the Master Levels, and Final Doom ($9.99 / €8.99)

How to use source ports with games downloaded from GOG.com
Some source ports automatically check the system registry for GOG's install folders, and will search it for the game IWADs. These include:
 * Chocolate Doom
 * Eternity Engine
 * GZDoom
 * ZDoom

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, or ) should then start the game.

Alternatively, the IWAD (named for Doom II,  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.