Shade

The author's tag-line for this game is "A one-room game set in your apartment," and it's difficult to say much more about it without spoilers. Suffice it to say that it's extremely well crafted and very, very creepy--this is "mess with your head" IF par excellence. Precisely what happens, particularly at the end, is open to multiple interpretations. There's also a lot going on, in a sense, so you may want to replay it a few times once you're done. Not to everyone's taste--it's not an upbeat game by any means--but masterfully done.