Well, according to my cousin the dental hygenist, your average over-the-counter kit is usually composed of zinc oxide and clove oil. You just mix them together, apply to the tooth, and wait for it to harden.
But the problem with these
DIY kits is that they are only supposed to last for maybe a week or two at the max. Their job is to simply plug the hole in your tooth until you can get a permanent filling put in.
Here is a link to one of the OTC kits currently available:
http://hocks.com/hocks-healthcare/hocks-product/A409441.html