I would like to list the following (all available on GOG):

Devil Daggers Forgive me Father GTTOD: Get To The Orange Door POSTAL: Brain Damaged Severed Steel

I fucking love devil daggers a great wake up game to play.

Not sure exactly what OP wants if they list 007 games, NOLF, Prey, and C&C Renegade, and Psi-Ops, but rejects Soldier of Fortune, Kingpin, Delta Force, and Judge Dredd. I know "popular and well-known" and "overlooked" are subjective, but we aren't exactly listing Halo, Half-Life, Doom, Duke Nukem, Quake and such.

And if we're talking about "well known", should we even include games that were covered by popular YouTubers? Hell, Civvie's TekWar video has 1 million views.

While I can't speak for the OP, I guess, it may have something to do with this sentence within his initial post: Going by that sentence, I'm assuming he only wants to list games that he doesn't have in his collection, already.

Not an expert, just suggestions.

NecroVision
Hidden & Dangerous series
Ion Fury

Here's another bunch of FPS games that all fall under the "mecha/giant robot" category (you decide if they qualify):

G-Nome
Krazy Ivan
Great Qin Warriors
Ultrabots
Iron Assault
Battle Engine Aquila (it's on GOG)

and another one in third-person perspective: Gun Metal (also on GOG)


Other TPS games you might consider:

Dark Void
Hydrophobia
Cold Fear
Velvet Assassin


Finally, Eradicator is another obscure FPS (available here) you might want to add to the list (actually you can toggle third-person view).

Microsoft's Urban Assault is a hybrid First Person Shooter/Real Time Strategy. It's pretty neat, but also has the Microsoft logo baked into the intro. Which would make a re-release tricky.

<span class="bold">GunMetal</span> is a 1998 last gasp for DOS futuristic hovertank combat FPS, It's a strange little game set over many levels.

Any of the SpecOps games other than The Line: https://www.mobygames.com/game-group/spec-ops-series
Early ones are tactical FPSs though I imagine a few newer ones are TPSs.

EYE: Divine Cybermancy: well known unknown FPS with RPG elements. Draws inspiration from Warhammer 40K and Deus Ex too I think.

Precursors: a less well known unknown FPS, looks like No Man's Sky

Vanquish: if Metal Gear Rising was a TPS

PositronX: a recent sci-fi shooter I've seen literally nowhere but on its release platform

Thanks, i added the SpecOps series, EYE Divine Cybermancy and PositronX to the list.

Thanks, added [Eradicator] to the list.

Thanks, added KISS to list.

Sorry if these have been added already:

Legendary

Marlow Briggs

E.T. Armies

Alien Rage Unlimited

You're welcome! And thanks for making the list, its a great resource for finding games you may have missed. Didn't see Precursors being suggested in the first reply, sorry about that lol. Vanquish is certainly out in the open and quite known but its fame pales in comparison to Paltinum Games' other creations, hence the suggestion. Also strange for Japanese studios to make shooters, even though this one does have hack and slash elements.

Steam sale for these 4 games at $0.59 each. Had these on my wishlist for awhile and waiting for a good sale.

Operation Matriarchy
Dusk 12
The Stalin Subway
The Stalin Subway: Red Veil

I have a last couple of potential candidates for the list: Starship Troopers (the one from 2005) and Aliens: Colonial Marines.
Both movie licensees, and both... widely regarded as bad games, actually. I don't know if they're really overlooked... well, maybe they are because they're bad.

However I'd like to thank all contributors to this thread. I discovered several titles I previously didn't know. Best of all, that KISS game that I think I had come across before, but not taken seriously (I was probably thinking of something along the lines of Iron Maiden's "Ed Hunter"...). I wish it was on GOG.
Then a bunch of other games such as Area-51, Scorpion: Disfigured, Red Ocean, BlackSite: Area 51, Übersoldier, Bet on Soldier: Blood Sport, The Mark, Precursors, Alpha Prime, and Marlow Briggs.
None of them could be called an unrecognized masterpiece, for sure, but they all seem fun, interesting, and well-made enough. Some are on Steam but I don't like buying there, even though they all come cheap. And none of them, I think, is worth the trouble to go hunting for a used copy, even though I'm sure they can be generally obtained for little money.


That Operation: Matriarchy looks strangely intriguing despite the less-than-stellar reviews...
Stalin Subway doesn't seem too bad. But Dusk 12, at least from what you can gather, just isn't good.