Baldurs Gate 1 and 2, including their various expansions.
Elder Scrolls: Morrowind or Oblivion, if you can run it. Or if you care to go further back, Daggerfall, or even Arena which you can get free, and LEGALLY so if you care to look around.
Vampire: The Masqurade: Redemption isn't bad.
Vampire: The Masqurade: Bloodlines is also great, if your computer can run it.
Arcanum is decent, though theres some bugs in it - though they might've been ironed out now. I never got to play with any patches or anything.
Wait around a month or so and Jade Empire will be out on the PC.
Dunno how it'll hold up, but it was pretty good on the Xbox.
S'pose I should mention both of the Knights of the old Republic games, they're on PC too right?
Hrm... I've heard mostly good things about Titan Quest.
I could go on forever, but last I'll mention is Freedom Force: The Third Reich.
Most of the above you could probably pick up cheap now, too. So woo.
