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.
Shodor Education Foundation
Shodor Interactivate: Lesson: Geometry in Tessellations
This upper elementary and middle school lesson plan explores tessellations and their geometric properties. Plenty of interactive support is available for students. Lots of resources for teachers to use.
Cool Math
Coolmath: Coolmath4kids: Tessellations
Tessellations are explained and described at this website. Three basic rules are given for creating tessellations.
University of Cambridge
University of Cambridge: Nrich: Tessellating Triangles
The challenge at this one page website, involves tessellation and triangle. Should you accept this challenge, you will find helpful teacher notes and the solution available right at the website.