F4LL0UT: Some more for your list:
Kreed - Sometimes dubbed the "Russian Doom 3", rather bad but a cult classic in Russia
Nitro Family - a totally crazy Serious Sam clone using the same engine
Devastation - IIRC one of the last major Unreal Engine 1 releases, incorporates some typical multiplayer features into an SP campaign
Vivisector - a weird shooter where you can shoot off lots of pieces off your enemies
Gore: Ultimate Soldier - interesting game, a little Quakish, a little Serious Sam
Bet on Soldier - sequel to Iron Storm where everything has to be paid for (weapons, armor, ammo, comrades and even checkpoints)
Firestarter - don't know much about it except that it's gory, crazy and by GSC Gameworld (Stalker)
Codename: Outbreak - tactical shooter with big open areas
Deadly Dozen - pre-Medal of Honor squad based WW2 shooter
NFK: Natural Fawn Killers - gory parody of hunting simulators with CUTE animals that you can shoot the heads off
Sniper: Path of Vengeance - another small and bad plot-based game
Pył (which means dust) - one of the last and prettiest DOS FPS games (from 1999!) with some very weird gamepla
Dead Hunt - "small" Serious Sam with tiny arenas but hundreds of enemies - very enjoyable!
World War Zero - a console port of Iron Storm which was then ported back to PC - weirdest "remake" ever - better in some aspects, worse in others
Boiling Point: Road to Hell - forgotten game in the style of Stalker that was panned for its many bugs - big world, RPG-features, vehicles etc.
Gunman Chronicles - not that obscure but forgotten - the one Valve game you can't get anyhwere these days and began as a mod
Outlaws - western FPS by LucasArts with great music and more realism than most shooters of its time (may have introduced manual reloading)
You can find footage of some of these games on this playlist (which you've probably already seen):
http://www.youtube.com/view_play_list?p=2BBA1AB1CF53EEAC Edit:
Oh yeah, and an impressive Half-Life multiplayer mod you may not know:
The Specialists 2.0
It was quite popular bet never got the fame of Counter-Strike or even Natural Selection. It's basically Max Payne as a multiplayer game (and makes it ridiculous that some press praised Max Payne 3's MP as a big achievement). This game had done this ages ago in its own original way. It has lots of weapons with real-time ballistics, slow motion, shootdodges and huge amounts of weapons. IIRC its bullet time has a similar concept to Max Payne 3 in that slow motion tries to only affect those players who are nearby and can see the others use.
dude, i racked my brain to come up with 3 fps's he might like. how in the world did you FIND all those?