already mentioned fallout as well as Planescape torment (the backgrounds. they are frigging beautiful and will always be...you need a mod to enjoy them tough)
Master of Orion 2
HL1 after a decade, after a sequel, after thousands of other games it is still one of the best fps out there.
Transport Tycoon. Jesus. game is as much fun as 15 years ago.
Civilization 2.
Nox
Red Alert 1. sure the gameplay is primitive but it is so much fun!
Adventure games. They don't really age. Teenagent, Beneath a Steel Sky, Monkey Island, Grim Fandango, the Dig, Indiana Jones, Longest Journey, broken sword, etc.. the time passed didn't really change how much fun those games are.