Well i've been gaming on pc for the most part of the last 10 years. Now some of the games i mention may be relatively old, but they are such awesome games.
Ok, first off we'll begin with fps/action sort of games. If you're looking for multiplayer games - and by the sound of it you're not into twitchy/realistic fps games like mw2 or BF:BC2.
Team Fortress 2
That being said i would advise you to check out Team Fortress 2. Its very much like quake games in sense that its not based upon hiding/sniping sort of thing and twitchy reactions. Its pretty much just run-and-gun.
Unreal Tournament 3
This is also a great game, its recent expansion was the Titan pack which added a new Greed mode. Greed is such an addictive mode, specially when you play with alot of people.
Half life series
This is notoriously one of the best fps games ever. Though mostly based on single player, it does also have a multiplayer mode. If you do go for this, definately buy the Orange Box value pack - by far the best deal ever! You get a ton of great games (which includes the whole halflife series, cs and a bunch of other games).
Left 4 Dead 1 & 2
Some have already mentioned this, but its a great game. Unlike the other games i mentioned its mostly based upon coop (similar to borderlands). Killing zombies is fun though i own both
Ok so moving onto games more based around single player FPS. I'd Strongly advise you to check out:
Mass Effect 2
This is more of a mix of RPG and shooter. ME2 is a spectacular game - i mean its got aspects of sci fi, great storyline, great characters, imaginative universe, very fun weapons to select from and overall its just an awesome game. Also from what i hear, is that they will be coming out with a multi player mode - some say it could be an expansion to ME2 some say its for ME3 which is supposedly coming out next year.
Another awesome shooter is Deadspace. Its single player completely, but oh my god! The experience you will have in this game is just crazy. Its like a survival horror/fps game set in space. It has you starting the game with you answering to a distress call from a ship. You go there only to realise that its heavily damaged. Going in with a rescue team you soon realise that everyone on the ship is dead. But as you look for clues as to what happend a deeper plot kind of unravels. And you gotta fight your way through dark corridoors full of mutated aliens. I very much nearly soiled myself while playing this game
Amazing experience none the less.
In terms of MMO's i'd stronly advise EvE online
If you rather not read anymore
just watch this trailer - says everything about eve in that on video:
YouTube- EVE Online - Butterfly Effect Trailer HD
Tens of thousands of players with a myriad of solar systems to explore. You got hundreds of different spaceships, pirates, wormholes to explore, massive pvp playerbase. Plus you can actually play for free - there is a monthly fee of $15 sub but you can buy ingame time with ingame currency. You'll 100% be able to save one up after a couple of months of casual play - some do it even quicker. I've been playing eve for about 9 months now, only actually paid for 2.
Anyway, i've rambled on enough
You might wanna check some trailers out for more insight of the games.Edited by Laten - 6/8/10 at 8:09am