Crysis. Very good game for 12 hours of fun crawling around bushes followed by 45 minutes of bewilderment followed by 5 hours of wishing you were back in the bushes.
The Crysis sequels very good for 16 hours of fun and has a better end mission than the original.
Crysis ran fine on my 2 year old PC. But then I'm not bothered about AA, shadows and water reflections. I couldn't really tell the difference in graphics quality with them on or off so just left them off.
Dead Space - this is too scary for me to play with my 6 year old son in the same room - but my 5 year old daughter loves watching me play this. Not as good a game as Crysis.
Multiplayer FPS - I've not found anything better for gameplay and variety of maps than the original 100% free Wolfenstein: Enemy Territory.
RTS multiplayer - I like Supreme Commander: Forged Alliance best. Great interface and good gameplay.
The Hearts of Iron series is good for a bit of WWII strategy, but a bit too easy to win and a bit too time consuming. I conquered the world as Italy. I started playing as Brazil to see if I could conquer the world with that country too, but I lost my saved game when I rebuilt my PC.
Rollercoaster Tycoon series is a bit of fun for a few hours.
Call of Duty series is good but rather linear.
I'll have to check out Far Cry 2. I enjoyed the first Far Cry but it sounds like the 2nd one might be another Midwinter (too much boring travelling around)?
The Baldurs Gate and Neverwinter series are enjoyable if you're into D&D.
The Pro Evolution soccer that I bought (version 7 or something like that) looked promising, but didn't have the appeal of the league / world cup tournaments of EA footie on the playstation, plus the interface for Pro Evo sucked on the PC.
And then a great alternative to computer games is a good boardgame, such as Agricola, Puerto Rico, Power Grid, Settlers of Catan, Victory in the Pacific, Speed Circuit.