Ahhh, I LOVE old games, because theres the fun factor that keeps bringing you back, with the added joy of all the good memories associated with them.
Every couple of months, I find myself playing through Fallout and Fallout 2 again. Easily my favorite RPGs ever. I can't wait for Fallout 3! Its being done by Bethesda Softworks, which is the same people who did Morrowind and that entire series, so I know that its in good hands...
I also play some from when I was little, from time to time, from the Super Solvers series. Most notably Gizmos and Gadgets, so simple, but so much fun, Number Munchers and Operation Neptune. And just today I reinstalled Heroes of Might and Magic 3. Classic game that never gets old (to me atleast). If you like that series, then be aware that a
Heroes 5 is in development. (yay!)
Another one is Gazillionaire, very fun trading game in space. Thats pretty much it.