Tessellations.org: Check out Tessellations.org for additional information on creating tessellations. Be sure to visit the online galleries to view Escher-like tessellating pieces designed by students. The site also hosts several different methods for creating these tessellating pieces.
Bookmark this online tessellation site as a wonderful resource for your tessellation unit. It would be a great introduction to an Escher unit as well.
M.C. Escher Official Website: http://www.mcescher.com/
Tessellation World of Makoto Nakamura: this artist creates colorful Escher-like tessellations, then animates many of them. This is an incredible site not to be missed in any discussion of Escher and Escher-like tessellations. Check out this Tessellation World for great inspiration. You'll be astounded at both the art and the mathematics inherent in these designs.