Baldur's Gate 2, at least once a year. I recently found my old Grandia discs, so when I get back to uni I'll see if they still work. I rock out on D2, SC1 and WC3 fairly frequently as well. I play some Armored Core 2, some of the PS1+ Final Fantasies, Zone of the Enders (1+2)...
I could go on. Don't think I don't adore new games, I've invested many an hour into Mass Effect, Dragon Age: Origins and will invest many more into Mass Effect 2, but that doesn't stop the old games from rocking my socks off seven nights a week.
Oh, and Sparkster: Rocket Knight Adventures for the Genesis. Admittedly it's emulated because my Genesis is at home, but I own both game and console, so I feel justified in emulating it, even if it's technically illegal since I didn't rip the BIOS myself.