Graham's number

Graham's number is the most unbelievably kewl thing I've learned in a while. Also called G64, it's a number so enormous it could blow your mind.

  Here's how the up arrow notation works.  (It's ... nested iterations of nested exponents.) Even G1, the very bottom layer, is gargantuan beyond imagination.

Check this out.  We know that the last 10 digits of G64 are 2464195387.

Learning about Graham's number gave me shivers of delight and awe.  And Knuth's up arrow notation is pretty damn kewl too.