well, with that in mind. I'll tell you some of the games that I have that I've enjoyed (for pc).
Starcraft battle chest (20 dollars, includes original and expansion)
Diablo battle chest (about 40 dollars, includes 3 games)
Command and Conquer : generals (I wanna get zero hour but not that badly)
WarCraft 3 : The Reign of Chaos / WarCraft 3 : The Frozen Throne, completely awesome IMO
Jedi Knight : Jedi Academy is very fun ( I still play with my friends alot)
Serious Sam was very fun, its a crazy first person shooter with really nice graphics for its time (I think its like 2 years old? I forgot)
BattleField 1942 is a cool series (especially when you get the Desert Combat mod for it keke)
Half Life : Counter-strike is an oldie but goodie
for starcraft , warcraft, and diablo, go to www.blizzard.com for info
for counterstrike, go to www.counter-strike.net
for command n conquer// battlefield , I guess go to www.ea.com
for jedi knight : jedi academy, www.lucasarts.com/products/jediacademy
for Serious Sam : www.croteam.com
as for newer pc games, I heard max payne 2 is cool. But I know its not multiplayer (all the games I listed above have both multi and single player capability). Deus Ex : Invisible war is supposed to be really cool (but I'd get more info first).
what pc games do you have already, so we dont recommend one of them hehe