Of the older games I have on my comp, the most-played is Star Trek- Elite Force. It was one of the early Quake 3 games and had pretty good graphics for its time. The gameplay is excellent, and the single-player story is quite fun to play through once in a while. Good multiplayer, loads of mods, and a decent role-playing community kept it alive for quite some time, and it's still played today on a small scale.
Others I've played and still play:
- Crimson Skies
- Metal Gear Solid
- Mechwarrior 4 Mercenaries
- Midtown Madness 1 & 2
- Motocross Madness 1 & 2
- Dark Forces 2- Jedi Knight
- Star Wars Episode 1 Racer
- Wish I hadnt gotten rid of Tony Hawk's Pro Skater 2
...and about half a dozen Game Boy and Game Boy Color games that still keep me from dying of boredom during long waiting room sits.