There is an attempt at categorizing them according to sub topic as much as possible. I am still adding to this list so if you know of other sites that aren't here, let me know. And come back regularly as I will be adding new sites often. At this website there are over 100 recipes for things as mundane as play dough and silly putty, to more goopy concoctions like slime and gak. You will also find recipes for crayons, clay, paint, dyes, bubbles, chalk and much more.
Some websites are just plain cool.
If you've got a budding engineer in your house, this site is a must. If you just like to fool around with animated soda straws linked with springs, you gotta go check this out. You can play with the constructions on the site or make your own. Either way, plan for the whole family to be there a while. This site will fascinate all ages over 5, even those who don't love science and math.