I made an account after reading this thread because this disscusion is right up my alley.
This is a long list by the way.
In no particular order:
This is a difficult list to put together by the way, there is a very fine line between obscure, classics' and
"cult classic"
Also note that I do not know ALL of them, just A LOT of them
Grim Fandango
Psychonauts
Beyond good & evil
X-Com
System Shock 1 & 2
Balder's Gate 1 & 2
Dungeon Keeper 1 & 2
Sam and max: Hit the Road
Star Wars Jedi Knight: Dark forces 1 & 2
Fallout 1 & 2
Space Quest
King's Quest
Ultima Games
Diskworld 1 & 2
Painkiller
Master's of Orion 1 & 2
Phantasy Star: Online
Mother 1 & 2 (aka: earthbound 0 & Earthbound)
God Hand
Killer 7
Eternal Drakness: Sanity's Requiem
Timesplitters: 1, 2 & 3
The Chronicles of Riddick: Escape From Butcher Bay
Jet Grind Radio and Jet Set Radio Future
Soul Blazer trilogy
Blood
Deus Ex
Neverwinter Nights 1 & 2
Myst (even though they haven't aged well at all)
Clock Tower games
Tribes
Call Of Cthulhu: Dark Corners of the Earth
Wing Commander 1 & 2 and it's spinoff
Privateer
Legacy of Kain (maybe)
Monkey Island 1-3
Day of the Tentacle
Vampire the Masquerade: Bloodlines
Ico
Planscape: Torment
Betrayal at Krondor (this game DEFINES the the term cult classic in gaming, if you haven't heard of this game, shame on you, everyone who has played this game considers this game the BEST rpg ever made in video games)
That are some games missing or some games on the list that aren't considered cult classics.
I might add some games later if I feel like it but it's hard to list games that aren't so much cult classics but are just "classics".
Edit: Zork is a good one.
Clive barker's undying