Cheap Arm & Hammer Baking Soda and unrefined salt. You can add a few drops of pure food grade peppermint oil for flavor.
There's this product on the 'net called "toothsoap". It's pure soap, with flavoring - orange or peppermint. I've tried it, but the soap gives me indigestion (even though I never swallowed it) every time I brushed, so I switched to plain old A&H baking soda. My daughter uses Toms of Maine w/o fluoride.