Quake 4 - You play as a marine, and other fun things, i wont spoil it... very rewarding gameplay
Doom 3 BFG - Its Doom with the expansions and crazy weapons on mars, quite fun

Unreal 2 - exciting, crazy weapons very sci-fi and a decent story
PREY - you get abducted into space and its your job to free your tribe and do other things i dont want to spoil

DeusEx Human Revolution - best game ive played all year, it was, is, and continues to be my favorite game sofar.
Red Faction: Guerrilla - open world, GTA meets mars with physics and destruction, easy game very long and fun

Red Faction: Armageddon - linear world with destruction, classic gameplay the most like Half-Life level of play.

The Chronicles of Riddick: Escape from Butcher Bay and the sequel, Assault on Dark Athena.
The greatest game ever created by man, its insanely fun, perfect pacing and the story keeps you interested from start to finish. I cannot recommend this enough

Maybe the Serious Sam's very fun very high paced action...

honorable mentions, and more what your looking for i bet:
Hard-Reset
Singularity
TimeShift
BulletStorm

Pretty much...I think I would also add "time travel" to that list

i wish they would continue with the Blood series.... Blood 2 was epic but full of bugs :P
and how come games like timshift and singularity are not advertised or famouse enough, the previews are amazing...
and y is my Rep back to zero?

Xenus 2 or Precursors ?

Starkrun mentioned Riddick a few hours ago, I'd like to second that. It's got really big guns, namely his arms, and nothing beats hearing the crunch of a guards neck.

Ooops shit, you already played it. Sorry, missed that post.

Needler, plasma grenades, legendary difficulty. Can't forget something like this, totally mind blowing.

I've never played Halo as I've always been under the impression it was primarly multiplayer? Is this true? Maybe I should give the 1st one a try if there is any kind of SP at all.

They do all have single player campaigns, I believe that they only ported the first two games onto the pc. I remember something about Halo 2 being vista only when it came out though this could be to do with directx 10. Which on a window vista or greater system won't be a problem now.

The original one is shit, to be honest. The destructable environment thing is a gimmick they used to sell the game in demo versions and stuff. It gets binned by level two or three, after which you rampage around indestructible corridors or have so few explosives that you couldn't really get anywhere even if you wanted to. Also, there's a one-shot-kill weapon called the Rail Driver that can shoot through walls. It's given to a whole hectacre of enemies at one point. Yes, they can kill you in one shot as well. No, they can't shoot through walls as far as I can tell (through cover, yes) but they're always on the lookout for you so you're pretty screwed either way.

EDIT: I would like to suggest the Marathon trilogy. It's brilliant.

Story mode is ok, better than most of that fps shit they dump at you today.

Terra Nova as in Strike Force Centauri?

Boy I loved that game. Absolutely utterly fantastic gameplay... and could be hard in times. Still very interesting missions, funny video acting... and at the end you get a kill score how many innocent trees you slaughtered.

dont play halo unless u have an xbox, i played the first two but couldnt play the rest...
space hulk are space marine from warhammer.... epic RTS game by the way i think i may try it... ok if u told me terranova and space hulk and who ever gave me old 3d fps game, i suggest that u download the DOSBOX and play the DOOM 2...

Yes, it's truly excellent, but not complete as such. It's missing the Xen part of the original Half-Life. It's coming, but considering how long the development time was on the first part of the game, I wouldn't hold my breath. So if you don't mind that it doesn't contain the whole of the original game yet, please do get it. They truly did an excellent job.

Off topic, and older, much older, how about Arx Fatalis? I know it's not sci fi but I need something to top off my list here and it is an FPS, kind of.