I grew up with Spirograph. Evan recently posted about the Spyro Gyro Pen.

Now, here's a great DIY project, provided by Paul Bourke, for the truly hard core geometry fiend. Try building a harmonograph. This contraption used to be home entertainment at the turn of the last century. It works by using two swinging pendulums which control the motion of the pen; as the swing slowly damps. the spiral pattern emerges.

Just remember, everyone, math is cool.