I have a love for strange indie games/mods and I just saw this on Rock Paper Shotgun and had to try it. It's quite surreal with an amazing narrator. It has a bit of a feel of Portal where you know there is something terribly wrong behind the plush carpet and plain walls.
http://www.rockpapershotgun.com/2011...anley-parable/
What makes this game shine is the narration though, and how it affects gameplay. The narrator is telling a story as you play but you can chose to do things contrary to what the narration is saying and break the story. IE: "As Stanley came to the end of the corridor, he knew that the staff cafeteria was on the left so he took the path to the left". I walked right.
You don't even need Half-Life 2 to play it, you just need the Source SDK which is free IIRC.