Mathemagic

While I understand that what this guy (Arthur Benjamin) does is mainly just a feat of memory, it's still damned impressive.

The most interesting part is the last few minutes, where he squares a five digit number, while thinking out loud, so you can see the algorithm he employs ... basically it's (a+b)^2 = a^2 + b^2 + 2ab, with some word mnemonics (ie, a peg system) to reduce the amount he has to remember.

Even if you're not into such geekery, you may enjoy the entire video, because he's not a bad showman.

Thanks to Spluch for the link.