Math Fun Facts

The Mudd Math Fun Facts! site is worth checking out. Although it doesn't detail the demonstrations for the facts exposed, it is quite mind teasing. I especially like the random fact page.

PlanetMath is also interesting for math-curious people. It is similar to wikipedia, but focused on math definitions, theorems and proofs. The presentation is pretty nice, using images (rendered from LaTeX it seems) for the formulas, but unfortunately the site doesn't load very fast.
Wolfram's MathWorld is another good reference.