>Progenitor of FPS games
Maze War
>First vehicle-based FPS, first FPS with distinct 3D visuals
Battlezone
>First FPS with Deathmatch, first FPS on a handheld
MIDI Maze/Faceball 2000
>First FPS with raycasting
Hovertank 3D
>First FPS with texture mapped walls
Catacomb 3D
>First "First-Person Immersive Sim" game, first FPS with texture-mapped ceilings and floors
Ultima Underworld: The Stygian Abyss
>First FPS with actual guns and graphic violence
Wolfenstein 3D
>First FPS with significant elevation/verticality in environments, first FPS with a strong modding community, first FPS with online multiplayer on both PC and console, first FPS with co-op gameplay
Doom
>First FPS with vertical aiming and jumping
Heretic
>First FPS with actual 3D texture-mapped environments
System Shock
>First FPS with crouching
Star Wars: Dark Forces
>First FPS with mountable vehicles and texture mapped polygonal enemies
Terminator: Future Shock
>First FPS with significant use of scripted sequences and interactive environments
Duke Nukem 3D
>First FPS with full 3D texture mapped 3D graphics and 3D accelerator compatibility
Quake
>First FPS's with a sniper-rifle and headshots
Team Fortress and Outlaws
>First FPS with real time limb-based damage
Goldeneye 007
>First FPS with large outdoor environments that weren't compromised by limited draw distance
Unreal
>First multiplayer FPS with significant implementation of mountable vehicles
Tribes
>First FPS with significant and effective implementation of stealth based around both lighting and sound
Thief: The Dark Project
>First FPS with significant use of scripted sequences for storytelling
Half-Life
After Half-Life I'd argue there were no new developments that really benefited the genre. Oh well.