People doing tessellations or interesting stuff

• Peter Raedschelders 
• Andrew Crompton
  (also people doing tesselation list, very great!)
• Hans Kuiper
• Christine Shearer
  (also take a look at the boxes) 
• John w Lefelhocz
• Escher rendered by Sacha
• David Scott Coen
• Hob David
• John langdon
• John Osborne

Great sites about tesselations:

• Totally Tessellated
• Escher Web Sketch  
  (Try it yourself)
• The World of Escher  
  (VERY NICE TESSELATIONS by CHILDREN)