I have played a fair share of shooters this year and will give you brief reviews of my favorites. Note that all of these games have great single player modes. For online you might have to get more info.
Prey - Amazing art, interesting weapons, neat special abilities, and a great story. This is pretty well a flawless game.
Quake 4 - A shooter's shooter with lots of weapons, the graphics are superior to most and the look and feel of objects is quite outstanding.
FEAR - Some of the best AI in any game I've played with nice atmosphere. The slow-mo ability is a nice twist too.
Call of Duty 2/3 - Fluid and chaotic action that happens all around you. A well polished game through and through.
If your into online FPS..then I have a couple to recommend..
Well obviously CS is one here..I play both 1.6 and Source but I like 1.6 better for some reason..Source is just so random at times IMO. It really depends on the player. I doubt you'll have troubles finding people to play with..
Unreal Tournament 2004 is the latest Unreal Tournament game out there..I haven't played much of it but apparently it's pretty cool. The vehicles is what put me off so I didn't buy it. But if your a fan of UT maybe it's worth picking up, alot of people still play it today.
I'm trying to think of some really popular online FPS but my mind has gone blank..but I'm sure if there was such a game available now I would of thought of it already. Stalker's multiplayer is pretty basic I've heard, that's disappointing 'cause I was really looking forward to it.
Battlefield 2 has just come to mind, I've played that online and that's fun..although I'm not sure how many people play it these days. I'm sure tons still do, also check out Battlefield 2142, never played it but I heard it's pretty popular if your into the Battlefield franchise.
I'll mention some worth taking a look for in the future..
-Unreal Tournament 3
-Halo 2 for Vista
-Team Fortress 2
-Huxley (MMORPG FPS)
That's all I can really think of right now..I hope this helped. I have no clue why I spent so long replying haha.