Oh there are way too many, and they're all original in different ways so you can't compare them. Here's my list:
Thief: The Dark Project - While it's not the only series of its kind, Thief is the first and by far the best. And back when every first person game was about killing the fuck out of every enemy you meet, a game centered on stealth is a welcome change of pace and also, I believe, the most original first person game of it's time. Portal is pretty fucking close, but it's lacking in a few places.
Katamari Damacy - Nuff said.
Silent Hill 4: The Room - You're trapped in your haunted apartment room, and you can't leave throughout the entire game, except through tunnels in the walls to other people's dreams. This game misses the mark in way too many places, but it still has one of the most original premises I've ever seen.
A few more, but I can't remember them right now. Most of them are flash games anyway.
Thief: The Dark Project - While it's not the only series of its kind, Thief is the first and by far the best. And back when every first person game was about killing the fuck out of every enemy you meet, a game centered on stealth is a welcome change of pace and also, I believe, the most original first person game of it's time. Portal is pretty fucking close, but it's lacking in a few places.
Katamari Damacy - Nuff said.
Silent Hill 4: The Room - You're trapped in your haunted apartment room, and you can't leave throughout the entire game, except through tunnels in the walls to other people's dreams. This game misses the mark in way too many places, but it still has one of the most original premises I've ever seen.
A few more, but I can't remember them right now. Most of them are flash games anyway.