The Mathemagician

There's a web site that posts lectures called TED.com.  It's goal is stimulate ideas:

"TED stands for Technology, Entertainment, Design."

One of the lectures is by a guy named Arthur Benjamin.  He can do some pretty amazing math in his head. If ever you need someone to figure out what the tip should be, he's the guy for you.

I think this qualifies a really nerdy, but I'll post it anyway:

http://www.ted.com/index.php/talks/arthur_benjamin_does_mathemagic.html

For more about TED you can read up here: http://www.ted.com/index.php/pages/view/id/5