Ever want to play Doom as BJ Blaskowicz? No? Oh. Maybe Duke Nukem? Of course you did. Well now you can! Samsara adds 7 new playable characters from other games contemporaneous to Doom. Duke Nukem, BJ Blaskowicz, Corvus, Parias, Security Officer, Chex Quest guy, and Ranger join Doomguy to battle the fiendish minions of whatever other wad you're loading it with.
For Zdoom and Zandronum.
There are also addon packs and a few forks of varying quality.