TF2 is about the best online first person shooter i think.
If you don't know about the actual gameplay, basically there's red and blue, and they hate eachother. So there's all these maps and plenty of user created ones, and you play games like payload, where the goal is to push a bomb on a cart to the end of the course within the time limit while the other color tries to stop you, capture point where there's points that you capture by killing the other color off and staying there without them long enough, and king of the hill, etc. etc.
There's a ton of awesome classes and you can find new weapons by doing awesome stuff or playing the class enough and good enough, or if you have a ton of extras of weapons you can combine them using crafting to build new weapons.
Of course you have engy who can build sentries, teleporters and dispensers, pyro with the flamethrower and the flare gun, demoman with a grenade launcher and the sticky gun, sniper who, well, snipes and throws jars of piss, spy who penetrates defenses and stabs people, scout who if devilish fast and can jump really high, heavy with the minigun and super slow running, medic who heals others and uses his syringe gun, ad scotch tape the wires (better than solder imo) who shoots rockets at people and stuff.
It's a really awesome game and I'm certain 999/1000 people here will tell you to get it.