The official M. C. Escher site.
Combinatorics Journal
Two articles:
R17, "Escher's Combinatorial Patterns" by Doris Schattschneider
R23, "On a Tiling Scheme from M.C. Escher" by Dan Davis.
Mathematica in Education and Research
A paper, "Automating Escher's Combinatorial Patterns", by Rick Mabry, Stan Wagon, and Doris Schattschneider.
Exploring Escher Tiles
Stan Wagon's pages on the Wolfram site, regarding Escher tiles.
Steve Ogden's implementation of Ellen Gethner's colouring algorithm
For Ellen's PhD she devised an algorithm for colouring patterns based on these tiles. Steve has implemented this in Java so that you can play it with on your computer. You must check this out, it's great!
Rui Menino's fantastic Eschertile program for Windows!
Rui has created a wonderful program for you Windows users out there. It allows you to create your own patterns and tile them in the same ways that Escher did. The site is in Portugese so here is the English google translation. I recommend this program to everybody who is interested in exploring this tiling method.
Hop's Escher Tiles
Escher-inspired tesselations by Hop.
Hop's Blocks
Mathematically inspired art and more.
Yoshiaki Araki's Escher patterns
An artist inspired by Escher.
Escher Pages
Neil Ziring's comprehensive Escher information site.