Tiling Plain and Fancy (Popularity: )
http://www2.spsu.edu/math/tile/ Tiling from the mathematical and historical viewpoint by Steve Edwards. Provides a number of examples of various types of tiles that tile the plane and includes periodic tiling plus Penrose's ...
Java Kali (Popularity: )
http://www.geom.uiuc.edu/java/Kali/ A program for drawing symmetrical patterns based on any of the 17 wallpaper groups, as well as several frieze and rosette groups.