System Shock: An adventure in aluminum trash collecting. j/k I remember playing System Shock in a dark room w/ lights flickering off the walls from the monitor...spooky. Alone in the Dark was spooky as well. *shiver*
My "laughed so hard I almost hurt myself" moment was playing Doom, original Doom that had the 2d sprites. 2d sprites weren't all that impressive, but it allowed you fight an insane amount of monters at one time on some levels.
There there was room with what seemed like hundreds of the pink bulldog monster things, which was locked.. Instead of the normal creepy sea of low grunts n' growls--I had replaced the normal low grunting noise with 'beavis 'n butthead's patented "Huh...Huhh..huh..huh" laugh. Pretty hilarious to hear it comming from so many pinkies..
HL2 is gonna be cool, and will probably kick ass but I think Doom 3 is gonna scare the pants of people.