If you want to lose your life.... take a look at civilization 3, Warcraft 3 (multiplayer) or Starcraft broodwar (multiplayer). Those are probably some of the most addictve...
Really old games that are good? Hmm if you didn't grow up with them you probably won't be able to stand 'going back in time' in gameplay and graphics but... memorable ones
RPG's:
Chrono Trigger (SNES, also on PS1, probably one of the best RPG's ever made, I'd love a modern remake of this puppy)
Final Fantasy 3 (SNES, also on PS1, it's named FF6 in japan)
Final Fantasy 2 (Snes, also on PS1, it's named FF4 in japan)
PC games:
Descent: Freespace and "freespace 2", in #2 they dropped "descent" from the title, Freespace 2 is a must though (it's a classic) the first one wasn't so hot (graphically/etc), but it got the job done.
Mechwarrior 2: Titanium edition (if you can find it) or the old Dos versions + Ghost bears legacy, this was just at the cusp of 2D/3D chips that could do texture mapping.
Need for speed: Porsche unleashed, probably the best balance between "simulation" and "realistic" driving game ever made, I have never played a game since that has the replay's/camera angles and car control as good as this game.
Planescape Torment - Do yourself a favor and PLAY this game, even if you do not like the combat, just going through it even with a walkthrough, is a MUST experience for anyone, IMHO. This is probably the best game on any platform with the most well done story in a game thus far.
Probably lots of others I'm missing, I'll have to think on it some more.