Totally Tessellated
http://library.thinkquest.org/16661/?tqskip=1&tqskip1=1&tqtime=0203
A superbly well-done site on tessellations. Winner in the ThinkQuest'98 competition. Features over 100 dynamic pages with quality illustrations, photographs, and links. Main topics: History of Tessellations, Essentials, Mosaics/Tilings, Escher, and Beyond. Background Information has links to polygons and angles, symmetry and transformations, regular polygons, plus links to a simple introduction to tessellations and to the effect of color usage on tessellated designs. Regular Tessellations contains links to real examples.

From the Math Forum:

• What are tessellations?
  http://mathforum.org/library/drmath/view/57675.html
• Tessellation Tutorials
  http://forum.swarthmore.edu/sum95/suzanne/tess.intro.html
  A good example of technology integrated into curriculum. A very nice section on where the math is in tessellates. Links to Escher sites.
• Investigating Tessellations Using Activity Pattern Blocks
  http://forum.swarthmore.edu/sum95/suzanne/active.html
• Historical and Geographical Connections
  http://forum.swarthmore.edu/sum95/suzanne/historytess.html
  Tessellations in history and architecture…

Tessellations
http://library.trinity.wa.edu.au/subjects/maths/tessel.htm
This site contains links to tessellations used in mathematics, art and design.

Mathematical Classification of Tessellations http://library.thinkquest.org/11750/eschpage/MathClass1.html
This site looks at the math shapes that Escher used and an explanation of Escher's use of symmetry in his art.

Alex Bateman's Origami Page
http://www.sanger.ac.uk/Users/agb/Origami/origami.html
Information, galleries and a program on making origami tessellations.

The Official WebOOG Home Page
http://www.mcmprod.com/applets/weboog/weboog2.htm
OOG, The Object Orientation Game, involves tiling puzzle games of Tangrams, Pentominoes, Hexagons and Polyominoes, which can be played at this site.