What's your favorite FPS game that many people don't know about?

My friend really enjoys Reflex and definitely considers it a lesser-known gem. I've been meaning to give it a try myself.

http://store.steampowered.com/app/328070/

Timeshift (got it from a magazine my brother bought in France, it's also on steam)

Tactical ops assault on terror (played this long ago and i remember enjoying it alot, got no clue where i got it from)

Gore: Ultimate Soldier (got this game on a CD from an old pc magazine, good times)

Dark Messiah of Might and Magic (it's a first person game, not really FPS, but you can shoot arrows and magic! Also best game experience right here, it's sad it never got a sequel that followed in it's footsteps)

There's probably a few more games to mention but i'm far too lazy to think about it. This should be enough for the curious.

EDIT: I wonder if anyone mentioned rogue warrior.

Delta force 1 & 2

I don't know if it is FPS ... but I loved Stranglehold

Counter-Strike 1.6

Cry Of Fear

Blue Estate

American McGee's Alice

CSI Solo missions and then yeti mode or something all enemy's became yeti ^^

Does Super 3D Noah's Ark count?...

Rainbow Six RavenShield + Athena Sword and Iron Wrath.

RavenShield

Prey, NOLF.

I just thought of another one; Hidden and Dangerous 2 and the expansion Sabre Squadron. It can be played in 1st or 3rd person though. I describe it as Rainbow 6 set in WW2. You control a small team of British special forces doing missions behind enemy lines. You can give orders on the fly or switch between them to get them in position for an ambush or assault. Some of the maps are pretty large and it have vehicles.

One of the most memorable missions for me was a two part mission. The first part you have to assault a village during the day and secure it in order to set up an ambush for an armored convoy during the second part. The 2nd part took place at night and you had a certain amount of time to prepare. I used some vehicles and traps to block off the road and force the convoy down a narrow street where I was hoping to disable the lead vehicle and block them in. I was using an enemy car to drive around and get supplies and set up the mines and traps. I was making one last run to pick up some stuff before getting into position. I had just finished loading into the car when we started getting shot at. The vehicle physics were very detailed even though vehicles weren't a main part of the game. The back tires were shot out and when I made a hard left to turn down a street for cover, I lost control and the car slid around and crashed. We had to get out and go on foot, being careful but trying to move fast as the convoy was supposed to be arriving soon. I don't know who was shooting at us, it could have been scouts for the convoy or some soldiers I missed during the first part of the mission.

My plan didn't quite work. The lead vehicle in the convoy was a tank and it just pushed through the vehicles I used to block the road. I set off my explosives, but it didn't disable the tank in the lead. It took out the 2nd vehicle, but the tank kept going. I ordered one guy to go with me leaving the rest to keep firing at the convoy. We chased the tank and I managed to hit it and disable it with a rocket just before it escaped. We then had to run back to the ambush site and finish off what was left of the convoy as they tried to back out and go another way, all the while worrying about the soldiers that shot at us earlier. No other game has ever gotten my heart pumping and adrenaline flowing like that game did.

I'll cheat. It's a TPS.
But as far as people not knowing about it, it's high up there.
When Ion Storm/EIDOS released the glorious Deus Ex and the infamous Daikatana, there was a third game that is almost completely forgotten today.

Anachronox.

By today's standards (and already at the time of its release), it's ugly as hell.
But it's a very fun game. Funny, clever and very entertaining. The combat system was somehow borrowed from the Final Fantasy model. The story is Mass Effect (but 10 years before Mass Effect) and you can play "spot the difference" chapter after chapter.
And have I said that it was funny ? Well, videogame humor, you know. Don't expect too much and you will be rewarded in spades.
Just like in Mass Effect, you will be assembling a crew, one member after the other. And one of the members will be a planet. A very small planet with people on it. That lives aboard your spaceship.
That game was nuts.

I didn't see this one mentioned yet. Mobile Forces!

I loved that game, it was kind of simple but had vehicles and I had a lot of fun with it at a LAN party with some friends. I even took it to QuakeCon one year and just happened to be sitting next to one of the Quake finalists... I got him to install it and we were playing together. Me and my friend still joke about it to this day.

I'm sure everybody has heard of these, but I had tons of fun with them too.
Red Faction
Soldiers of Fortune 1 and 2

Postal2

btw you can save up to 90%

http://steamcommunity.com/sharedfiles/filedetails/?id=504617099

or those:

http://steamcommunity.com/sharedfiles/filedetails/?id=505033373 :-)

Assault Cube

http://assault.cubers.net/

Hard Reset
Singularity
Wolfenstein (2009)

RAGE

http://store.steampowered.com/app/9200/

Alpha Protocol 8P

There was also Psy ops

Dirty Bomb

Homefront

NecroVisioN

Shogo: Mobile Armor Division

Sounds weird, but is 100% kick ass.