Talk about games such as Doom, Quake, Wolfenstein, Shadow Warrior, Duke Nukem, Marathon, Unreal Tournament and more!
Don't forget the Marathon Trilogy! (Marathon, Marathon: Durandal, Marathon: Infinity) And if it counts, the Fanon trilogy as well. (Marathon: Phoenix, Marathon: Rubicon, Marathon: Eternal)
Wait what?? What fanon trilogy?
Also, ever heard of Samsara? It's basically the crossover fighting game concept, but applied to old school FPSes.
(violence warning (obviously))
I played Doom a few times.
It was not fun.
Maybe because I suck.
Look up "Aleph One." It's basically a recreation of the Marathon engine with some changes to make it run better. It's also rather moddable, and plenty of people have made their own scenarios. The three that are widely regarded as the best fan-made games are Phoenix, Rubicon, and Eternal, and as they don't contradict one another too heavily story-wise, many people treat them as being part of the same continuity, as a sequel trilogy to the official Marathon games.
I've actually played Rubicon and Eternal, and can vouch for their quality.