Game of the Day: 20 Questions
This one is really freaky. A bot that you play 20 questions with, where it tries to guess what you're thinking of. It's freaky because most of the time it guesses correctly, even if you're thinking of something really obscure.
[EDIT: Yeah, I know this has been around a while. I'd forgotten about it, then discovered it again while cleaning up my bookmarks. If you haven't seen it, try it out. It learns more from every session. Example: Correctly guessed "Pool Cue" in 15 questions.]