Pentominos

This site investigates the four-color problem:
http://www-groups.dcs.st-andrews.ac.uk/~history/HistTopics/The_four_colour_theorem.html

A beautiful gallery of fractal designs are found on these sites:
http://fractals.hauner.cz/1

A site showing some of Escher’s tessellations is interesting:
http://www.mathacademy.com/pr/minitext/escher/

This site shows the colorful answer to Problems 24 and 25:… See the whole entry