Alien Cabal - Save earth from the aliens in this fun 3D shooter.

Blake Stone: Aliens of Gold - Impressive and deep shooter.

Doom - Some of you may have Doom 3 sitting around somewhere. See where it all started.

Duke Nukem 3D - In my opinion, the best DOS 3D shooter, worth downloading.  Great transition to three dimensions for the Duke Nukem series.

The Catacomb Abyss - 3D shooter released by SoftDisk in 1991.

Tomb Raider - Play the game that started it all.

Wolfenstein 3D - Very popular 3D shooter.  The PC ancestor of many more recent multiplayer games.