Okay, I know we were all kinda disappointed by the Song Tapper...but I promise this is better.
http://www.20q.net
My friend had a handheld version of this. You think of something, anything. It asks you up to 20 questions, then guesses what you're thinking of.
The first time I did it, I thought of a baseball bat. After 20 questions, it came up with cricket bat (I may have answered one question wrong). If you say it's wrong, it'll ask a few more questions and try again. After a couple more questions, it got baseball bat.
I just tried again, this time thinking of a carboard box. After just 18 questions it got "cardboard". I pressed it a little and by question 26, it got Box (cardboard).