Kali

http://www.geom.uiuc.edu/software/download/kali.html

Kali is a program that lets you draw symmetrical patterns based on any of the 17 wallpaper groups, as well as several frieze and rosette groups.

The large area on the right is the drawing area. To draw, put the mouse there and click the mouse button (if your mouse has more than one button, it shouldn't matter which one you click). This starts a line. Click again to end it and start another segment, and so on. To drop the line completely so that you can start a new line, click the mouse button twice on the same point.