I'd like to toss in a vote for everything made by LucasArts (especially Jedi Knight: Dark Forces II), both Age of Empires 1 and 2 + expansions and Serious Sam First and Second Encounters. Also the original Half-Life if you haven't played it yet.

What's nice about the original Serious Sam is although it was quite advanced for its time, it was not very demanding on hardware. It was sort of like a reverse FarCry or Crysis. You can run Serious Sam Second Encounter on Windows 95 OSR2. If you liked Doom or Quake at all, you should try Serious Sam.