Half Life I included for its influence on present day fps's.
It essentially waved in the second generation of the genre as it invloved more than just simply shooting random beasties and collecting keys. Its introduction of mods - in particular Counterstrike also makes it stand out. That said, its still a goddam fps which derives from Wolfenstein 3d, so hey could be lobbed off.
Planescape: Torment, ive never personally heard of, but its mentioned several times now. Guess these people can explain.
The Eidolon. A 3d environment game (although it probably uses a 2d trick also) by Lucasfilm in 1985/86 on 8 bit computerss - before Phantasy Star. A very early fps (before Driller even)
Way Of The Exploding Fist. This came out before and heavily influenced IK+. Its not so humourous, but its style is near identical and along with Kung Fu Master is one of the earliest beat em ups. Still, you could argue IK+ had mroe impact.