Castle Wolfenstein 3D (Singleplayer only) A castle overun by Nazis and their dogs. Classic shooter genre.
Doom 1993 By ID Software: The epic multiplayer shooter that began the wad (now mod) craze.
Marathon 1996 by Bungie: Some great level add-ons were made for this Mac only game.
More Recent Mulitplayer Shooters:
Quake, Quake II, Quake III by Id Software: Classic multiplayer series known for beginning lan parties and frag fests.
Unreal 1998 by Epic Games: An engine known for high resolution, lighting and special effects.
Halflife 1999 by Sierra Studios (uses Quake engine): Male geek goes postal in underground lab scenario.
Tribes 2000 by Sierra Studios: Lovely snowy mountain peaks that you can fly around as you shoot people.
CounterStrike 2000 by Sierra Studios: a multiplayer mod for Half-life with an uncanny "realistic" terrorist theme.