Age of Empires (I and II)
Assassin's Creed (all of them)
Battle for Middle-Earth (again, all of them - I'm even making a
mod for the first one!)
Chip's Challenge (REALLY old game, but I love it!)
EverQuest II (if anyone cares: Arlorielle, Laranelle, Teran, Jelanna - Freeport server; Dythar - AB; Dalnath - Permafrost)
Portal
Rome: Total War
Tetris (of course!)
And, though it barely counts...
Windows Solitaire. I swear I've spent more hours playing that then I have sleeping.