Every official Doom game and various other commercial games based on the Doom and Doom 3 engines have been released on Valve's content delivery system Steam. Most of the games were made available on August 03, 2007.
On Steam, classic Doom games and other games that use the Doom engine by default run under the DOSBox emulator, but it is also possible to use source ports with the games. More information on this below.
Classic Doom games
- The Ultimate Doom ($9.99 / 9,99€ / £5.99) 
- Doom II with Master Levels ($9.99 / 9,99€ / £5.99) 
- Final Doom ($9.99 / 9,99€ / £5.99) 
Doom engine games
- Heretic: Shadow of the Serpent Riders ($4.99 / 4,99€ / £2.99) 
- Hexen ($4.99 / 4,99€ / £2.99) 
- Note: Contrarily to all other Doom-engine games available on Steam, this version of Deathkings is not the latest and requires the to work correctly.
Doom 3 engine games
- Enemy Territory: Quake Wars ($14.99 / 19,99€ / £14.99) 
- Prey ($14.99 / 14,99€ / £8.99)  (not available in the UK, but can be activated via retail CD key)
How to use source ports with games downloaded from Steam
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 folder <Steam directory>\SteamApps\Common\<game name>\base. 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 <Steam directory>\SteamApps\Common\<game name>\base 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.
If you wish to run the games with a source port while via Steam or its shortcuts, you need to decompress and copy the source port files to <Steam directory>\SteamApps\Common\<game name>\base as instructed above. Then you need to edit a .bat file located in <Steam directory>\SteamApps\Common\<game name> (bat files can be edited with any text editor, such as the Windows Notepad). Doom II, for example, has three bat files, one of which is named "doom2 + mouse.bat". Open this file in the text editor. The file contains the following type of text:
.\base\dosbox -conf .\base\doom2m.conf -fullscreen -exit exit
You should then edit the top line to point to the main executable of the source port. For example, for ZDoom it would be as follows:
After this the Steam shortcut should launch the source port version of the game, as long as the game is launched by using the basic launch option and not the "Launch <game name> with classic controls" alternative.