David Joyce - 17 Wallpaper Groups

Steve Edwards Identifying the 17 Plane Symmetry Groups

David Reid Symmetry

Mathworld Wallpaper Groups

Michael Sheppard Big page of links

Geometry Junkyard Symmetry - including Indra's Pearls

Shodor Symmetry in Tessellations Discussion

NZ Symmetry Lectures -- pdf file with many examples and explanations