(what mathematicians do in their spare time)
(There isn't much on here at the moment as this mathematician hasn't had much spare time!)
Four dimensional graphing: see what happens when imaginary numbers are allowed in y=e^x. Animated graph included!
Dabbling in the Warp: a term paper I wrote way back in 1999 on non-Euclidean geometry. I make no claims as to its accuracy but I remember having a lot of fun writing it anyway.
(The following links are placeholders and don't go anywhere yet.)
To measure the earth: learn how Eratosthenes measured the diameter of the world 2200 years ago with nothing but string and shadow.
Bring me that horizon: using geometry to calculate how far you can see from the crow's nest.
Pascal's Triangle: useful for the Binomial Expansion Theorem, but full of other fascinating patterns as well.
Pascal's Pyramid: why stop at two dimensions?