Well, its not the most powerful, but most Source engine powered games would probably run OK, so:

FPS:
Left 4 Dead
Half-Life 2 & episodes
Portal
Counterstrike: Source
Far Cry
Deus Ex (classic)
Bioshock (should run)
Call of Duty 4

RPG:
Neverwinter Nights 2
The Witcher
Baldurs Gate 1 & 2 (old)
Planescape Torment (old)
Knights of the Old Republic 1 & 2
Oblivion (should run on mid-high settings)
Fallout 3 (should run on mid settings)
Mass Effect (not sure how well this would run, but should run OK on low-mid)

Strategy:
Age of Empires 3
Age of Mythology
Supreme Commander
Warcraft 3
All Total War games (although I'm not sure of required spec for Empire)
Civilization 4
Starcraft (old)
Darwinia/Multiwinia

Laptop gamer, good to see, I'll try to keep the games lower on specs!
Well this list can be very long but I am at work so here's a quickie ;)


Aquaria - PC (completely unheard of amazing indie game on steam)

Celestial Impact - PC (FREE open source game that blows away all 'destructible environment' games away)

Warsow - PC (FREE open source game that is even more twitch than quake 3)

Killing Floor - PC - was rather unheard of but it recently went commercial and is selling for $20 on Steam so it is doing rather well now. Still amazingly hardcore game, rarity these days.

Left 4 Dead/ Half Life 2/Team Fortress 2 - Just get the Valve Complete Pack!

Battlefield 2

Fallout 3

World of Goo

Quake Live (FREE classic quake goodness)

Tribes 2 Next (classic fun for free now!)

Legions: Fallen Empire (Free similar to tribes)