Arguably, the very first actual first person shooter was Wolfenstein 3-D in 1992. There have been hundreds of mods since created for this primitive game. For a wolf3d exclusive site, try the Wolfenstein 3-D Dome. If you would like a ton of wolfenstein 3-d style mods for DOOM II, visit Wolfendoom or Laz's. For some map editing toys, try Havoc's Wolf Editor, FloEdit, Mapedit for DOS. Other utilities here for wolf.

There were also games made with the wolf3d source sold as retail, including:
The Blake Stone Series, Corridor 7, Operation Body Count, Rise of the Triad, and Wolf3D itself had a sequel, Spear of Destiny. For mods made for some of these source clones, try Wolfenstein Goodies. There was also Ken's Labyrinth, written by the kid who later wrote the entire Build engine for Duke3D Also, my favorite, Nitemare 3-D. The spelling was wrong on purpose, as that's how the game was spelled. It was a sequel to the Hugo Trilogy which was an interactive fiction game with primitive graphics.

My very first venture into editing my own levels on video games began on the NES with Dudes with Attitude. When I got a PC, long before DOOM level editing, I found a few little games on a collection cd-rom of games called Snarf and Jetpac. Jetpack 2 is happening in modern day. Another cool 90's level editor would be for Nintendo, you know the game, Super Mario Bros 3. Before Mario Maker of the modern days, there were hack tools to completely rearrange this game for other players in the world to try out. For level editors of games 90's and beyond, check out this wiki entry on games with level editors. The ones I talked about of course aren't on here, meaning there are probably a ton more that didn't make the page either.