Math Cats http://www.mathcats.com/
A magic chalkboard takes you to a math art gallery and lots of interactive math activities, including: magic squares, conversions, seasonal surveys, symmetry, tessellations, geometric designs, games.

Mudd Math Fun Facts http://www.math.hmc.edu/funfacts/
An archive of interesting math facts for use in the classroom or just for fun. Browse by subject, difficulty, keywords, or try the "random" feature. Based at Harvey Mudd College.