I don't know what your taste in computer games is and don't presume to know either. I'm not too into the military sim type games like Soldier of Fortune II. I'm more of a casual FPS gamer.
If you're new to FPS here's what I would recommend:
-Half life. To this day, I still think that this is the best FPS out there because it has a strong community. Ever since Half-Life was released, it has had a larger online community than Quake 2, Quake 3, Rainbow 6, Unreal Tournament, Return to Castle Wolfenstein, Medal of Honor, Soldier of Fortune, No one lives forever, Tribes, and Tribes 2 combined. Those are the major players I can think of, any other multiplayer-capable FPS games can be added to the list. Go to www.gamespy.com right now and scroll down to the very bottom of the front web page. Last time I checked Half-Life had some 50,000 players. Medal of Honor, the closest competitor, has about ~3000 players on average +/- 500. The good thing about Half-Life is that Valve is very involved with the mod community. They don't post stuff and make promises or anything, they simply help the mod community. In other words, they are like the british royalty before they got into the media. When the game devs speak less and make less promises, it's funny, but you revere and respect them much more. For example one of their programmers does a lot of work programming installers for practically every major HL mod out there, but he doesn't put his name on the installers. Valve considers post-game support for the mod community as simply an aspect of supporting the game afterwards, much like you'd support a game with patches (I know cause I visited Sierra once). Plus they have an annual HL mod expo (this year it's in Texas) and they give a lot of individual attention to folks making mods for hobbies (heck that's how CS got commercialized in the first place). I'd say every week or two there is a new mod announced. There is also already a large cache of finished mods, inclduing many hours of single player levels. If you want more single player you can even buy two official expansions, opposing force and blue shift. Besides this it has popular mods like counter-strike, TFC (this is a blast to play as it is very casual and not as competitive), Day of Defeat, Frontline Force, Desert Crisis, Action Half-Life, and Firearms, not to mention bots for lots of these mods. There's more coming soon as well, including Hostile Intent maybe, a cell-shader animated mod, a worms mod, a medieval class-based mod, a racing car mod, and a sidescroller mod ala mario bros, with lots more in development and even some single-player episodes in development too. Finally last but not least, it has the mod <A HREF="http://www.svencoop.com" target="_new">sven coop</A>. Being a huge fan of coop and not so much the high adrenaline teamplay based games I like this mod a lot, and to my knowledge it's the only action game that has a comprehensive cooperative gameplay system besides serious sam. Sven coop has lots of good maps and hours of gameplay and in a week or two they are releasing their next version. This brings me to...
-Serious Sam. Never played it but it sounds like a blast. You can buy it on ebay for $4 or less. It's a steal. It has great multiplayer co-op too, and two guys who are brothers are working on an awesome starship troopers mod for it where you basically cooperatively fight as those guys and kill bug after bug after bug... (fyi half-life also has 2 starship troopers mods in development, one in <A HREF="http://www.starshipmod.de" target="_new">germany</A> and one in france[forget the url])
-Star Wars Jedi Knight II Jedi Outcast. This game already has a strong mod commmunity backing it, has a co-op <A HREF="http://www.raven-games.com/hosted/coop/index.shtml" target="_new">by the same author of this mod</A> mod in the works (I e-mailed the guy who is working on it), has fast-paced melee lightsaber battles, bright and fun graphics (no "realistic" textures or blood here), and best of all you don't have to be a star wars fan to enjoy it.
-Star Trek Voyager Elite Force. I bought this game as soon as it came out I remember, because it got such amazing reviews. Reviewers were all screaming, "It's the best damn star trek game I've ever held in my bloody hands!" Well it is pretty darn good. Surprisingly I liked this game not only for its excellent single player (you always had AI people following you around and helping you fight which is so unique and different that I don't think it can be compared to Half-Life's) but also for its bots. I spent hours fighting the bots in this game on my slow athlon 800.
Those are the first four I'd recommend to you. The truth is, multiplayer is where FPS games shine. And I really like these two games because, unlike all of those other 'amazingly innovative' multiplayer games, these two actually have real cooperative teamplay when you tire of playing 30 rounds of CS.
If you are into realistic military sim you probably won't like most of the games I'm suggesting to you besides Half-Life perhaps. These include SoF II, Operation Flashpoint, the Rainbow 6 series, the specops series, the delta force series, and all the rest.
This little cathode light of mine, I'm gonna let it shine!