Mathematical surprises

I would include the Diffie-Hellman key exchange algorithm in that list - it is pretty simple but wonderfully counter-intuitive:

http://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exch...

I've always liked the birthday problem:

http://en.wikipedia.org/wiki/Birthday_problem

That's great list, was glad to see some familiar ones there too. I'd suggest putting up for example your top10 list. It takes a long time to go through all of those and as mentioned, not all are that intriguing.