The Discontinuous Groups of Rotation and Translation
in the Plane by Xah Lee gives illustrations of the 17 different types of
Wallpaper designs.

A
Java Applet to play with Penrose tiles by Shuxiang Zeng is quite nice.

The Kali program at
the Geometry Center is an applet that allows interaction with the frieze,
wallpaper, and rosette groups.

Erich's Packing
Center gives diagrams of the current best-known packings for 10 circles
in a square, and other similar problems. One excellent little problem
as an example: Fit 7 equilateral triangles with a side of 1 into
a square with side 2. YOu can find the answer at his site.