Fabulous interactive on-line SpiroGraph from David Little.
I had a SpiroGraph set and used to play with it for hours but was often disappointed in the quality of my workmanship - too often a little circle would jump and my pen would skid or the biro would blob... so I've been delighted to play with this one. Its got lots of possibilities that a real world set didn't have too. The only thing you can't do directly in it is superimpose curves on top of each other, which was something I used to do using different colours.
Here are a few unusual curves:
Click on any to see larger.
There are other SpiroGraph sites on the web: gallery of multicoloured SpiroGraphs and another gallery.
Have you found any interactive versions of favourite toys on the web?