Here are some good ol' games that I remember spending a lot of time playing when I was younger:
MechWarrior 2 - I spent hours playing this game. Would play it right now if it worked on XP....
MechWarrior 2: Mercenaries - Equally fun as Mech2, and offered more than GB's Legacy did.
Zork: Nemesis - One of the darkest in the series, but by far the most interesting.
Zork: Grand Inquisitor - Funny and clever gameplay.
Bioforge - Not many remember this one, but it was WAY ahead of its time.
Baldur's Gate (Trilogy) - The most enjoyable D&D games ever made!
Icewind Dale (Series) - Equally good D&D games!
Myst - Love it or hate it; this game takes me back in a "good" way.

Total Annihilation - The most enjoyable RTS ever created. Awesome music too!
DOOM - You can't list the classics and not put this one up here....
Homeworld (Series) - The most impressive 3D RTS I've ever played. Wish they'd make more.
Caesar 3 - Best old-world city-building game ever because it was challenging and yet still fun!
Diablo (1&2) - Both classics. I hope #3 doesn't disappoint....
Age of Empires 2 - Balanced, fun, and addictive!
System Shock 2 - Creepy and full of awesomeness. Gotta love SHODAN.
