The other day I was looking up a PC version of Rings of Zilfin [http://www.mobygames.com/game/dos/rings-of-zilfin] just to remind myself how it was played - very different from RPGs these days. 1987 game - how's that for Old School? I actually found myself playing it a bit longer - it's still got appeal, it's a pity emulation can't do it justice. It's a real shame that the much better Commodore 64 version is thwarted due to copy protection and the graphically superior Atari ST version cannot be found anywhere.
Another couple big favorites of mine are Sentinel Worlds I: Future Magic [http://www.mobygames.com/game/dos/sentinel-worlds-i-future-magic] (1989) and Star Control II: The Ur-Quan Masters [http://www.mobygames.com/game/dos/star-control-ii] (1992). Yes, I'm aware that later was remastered and released in a free version, which reminds me of Ultima 7 [http://www.mobygames.com/game/dos/ultima-vii-the-black-gate] and Exult [http://exult.sourceforge.net/]. I'd probably drag these out more often, but I beat them all at least twice, and time tends to escape me too easily.
That said, I usually don't mess around with old games these days, though I've many a humble memory for my Commodore 64 days. The earliest game I can remember playing was actually Radar Rat Race [http://www.mobygames.com/game/vic-20/radar-rat-race] for the Vic-20.
These days, when I'm playing older games, it's usually some blast for the past recently re-released on GameTap. There's so many games these days that it's stunning how many I missed in recent years. For example, Gothic 3 was just released on Gametap - dang, I've been meaning to get around to that one. The whole Splinter Cell, Tomb Raider, and Hitman series? Right under the radar - only played the demos when they were new. Yeah, GameTap's a pretty great service to have for nostalgia and chances lost.
[Edit: Reading this thread, I can see some other good ones here that would apply to me. One of the first games I tried off GameTap was Startopia. Deus Ex was another game I finished at least twice. System Shock 2 sat on the shelf, having terrified me, for several years until I finally worked up the backbone to finish it some time last year. New copies of Diablo 2 are still selling in stores. Hero's Quest/Quest for Glory: Absolutely. The whole Monkey Island Series: Good idea. I actually reinstalled and finished Fallout 2 for the first time, 10 years late, right before Fallout 3's release.]