National Council of Teachers of Mathematics
The Math Forum: Tessellation Tutorials
This site provides a wealth of tutorials regarding tessellation. Content ranges from basic definitions of what tessellation is, to the math behind it, to various learning activities.
National Council of Teachers of Mathematics
The Math Forum: What Is a Tessellation?
The Math Forum defines and illustrates geometric tessellation.
Wolfram Research
Wolfram Math World: Semiregular Tessellations
This one page Mathworld site has a number of examples of semiregular tessellations. A definition and a small amount of related information is given at the bottom of the page.
Math Cats
Math Cats: Visit Tessellation Town on Tile Island!
Go to Tessellation Town on Tile Island and create a variety of tessellations.