1. Investigating Patterns: Symmetry and Tessellations

Good links, including to Escher sites, the 17 Wallpaper Groups
(near the bottom of page), and numerous other interesting sites.

2. Symmetry and Pattern: The Art of Oriental Carpets

Check out the About Symmetry and Pattern link, with the
"Four Basic Symmetries" (which are operations like
reflection and rotation, that may be performed upon a
region or design).

3. Quilt Gallery Magazine Technique-Tessellations

Note the Ohio Star link about 3/4 of the way down the page.

4. Kali

Interactive java application with 17 wallpaper groups.

For additional information on the 17 wallpaper groups, try these.

Wallpaper Groups: the 17 plane symmetry groups

Wallpaper Groups: transformations