It's pretty simplistic. I am not much into mech sims, but this played mostly like a 3rd person shooter, except with mechs. A bit consolish, but I had fun with it. Somewhat similar to Slave Zero I'd say

Two more games came to mind:
Chrome
Venom. Codename: Outbreak

Might as well contribute some more titles. Although I should probably ask is the OP looking for titles that are obscure or merely forgotten to time? Because I think some games were popular back then but now are basically a relic today. And does the quality of these matter, do they have to be good and not terrible?

Terra Nova: Strike Force Centauri
XS
Soldier of Fortune series, arguably
Kingpin: Life of Crime in the same vein
Delta Force series, including Black Hawk Down
Star Trek shooters
Judge Dredd: Dredd vs. Death

Is Shogo "Popular and well known" for you? If not, it's great.

How about Project: Snowblind, which was originally the 3rd Deus Ex game, but late in development redone into its own thing.

Thanks for your contribution. Games like XS is what i want to add to the list. I never heard of it before and its a FPS. I will also add [Star Trek: Klingon Honor Guard] which is a good fps that most gamers don't know about. The other games that you listed are all great games, but they are too popular and well-known for my list. Most gamers can find and buy them easily. As far as quality, i'm not picky at all, as long as its not complete trash. Thanks again for adding 2 game to my list.

Thanks for posting. Both are really great games, but is too popular and well-known for the list. Most gamers know about them and they can be easily found and bought. Games like [Blacksite: Area 51] and [Turning Point: Fall of Liberty] and [Jonathan Kane: The Protector] is what i am looking to add to the list. They are good FPS that most gamers never heard of and have trouble finding and buying. Thanks for the info about Project Snowblind, very interesting, i did not know that.

1st Person:

Mortyr 1+2
Übersoldier 1+2

3rd Person:

Insanity: Afterfall (problems with unpaid Unreal Engine royalties)
Insanity: Reconquest (problems with unpaid Unreal Engine royalties)

Aliens vs. Predator 2 is certainly both "popular and well known" and therefore should not be on a list that supposedly doesn't include popular and well known games.

I'm not sure I would call either the NOLF series or the Vitcong series "overlooked", or "unknown", or "unpopular", but that probably depends on whom you ask, of course.

However - there are two games missing on your list, that might actually meet your criteria:

1) Nocturne:
[url=https://en.wikipedia.org/wiki/Nocturne_(video_game]https://en.wikipedia.org/wiki/Nocturne_(video_game[/url])

2) The Devil Inside:
[url=https://en.wikipedia.org/wiki/The_Devil_Inside_(video_game]https://en.wikipedia.org/wiki/The_Devil_Inside_(video_game[/url])

Both are "horror shooters" in third person perspective.

Edit
Just thought of a third one that might fit your criteria:

3) Second Sight:
[url=https://en.wikipedia.org/wiki/Second_Sight_(video_game]https://en.wikipedia.org/wiki/Second_Sight_(video_game[/url])

Forgot to mention that Z.A.R. also has a mission pack to it with new levels, weapons, enemies etc. However, it was only ever released in Russia, has only Russian localization and can't currently be bought anywhere.

I never heard of the second game until it was mentioned here. I know the first game was popular and well-known, but alot of gamers don't know about the second game.

Yeah sure, GOG once famously used "a lot of gamers" argument which was similarly credible. Seriously though AvP 2 was like 10 times more acclaimed than AvP. If anything I would rather say that first part is overlooked which was totally overshadowed by sequel.
16k votes on wishlist: https://www.gog.com/wishlist/games/aliens_vs_predator_2

Wow, great thread, I'll make sure to peruse it later. For the moment, being sort of an FPS enthusiast myself (though I certainly don't claim to be an expert), I'd like to contribute to the list with some titles you might not be familiar with (although I can't know for sure):

Kreed & Kreed: Battle for Savitar
Devastation
Psychotoxic
Gore: Ultimate Soldier
CyClones
Iron Storm*
Redline*
Shadow Ops: Black Mercury*
Firestarter*
Chasm: The Rift**
Commandos: Strike Force (part of Commandos series)
Syndicate (inspired by the 1993 game)
Carnivores: Cityscape
Will Rock
Malice (commercial Quake mod)
X-Men: The Ravages of Apocalypse (commercial Quake mod)
Gunman Chronicles (commercial Half-Life mod)

(* = it's on GOG, ** = it's coming)

(please note that I'm not claiming these games are all good, actually some of them likely aren't...) This is the one I find the most interesting. Reviews are mixed, but the atmosphere seems to be top-notch.

The bloodrayne series and Sudeki comes to mind.

Great list, thanks for your contribution. I will update list when i get the chance.

I'm going to listed some console FPS need to be ported to PC someday.
Killzone series
Resistance series
The darkness 1.
alien resurrection