Here's a good article on clay litters:
I use World's Best for multiple cats, and I've never had any problems.
We would recommend Worlds Best Cat litter which uses all natural whole-kernel corn to deliver superior odor and clumping control. Flushable and septic safe making it easy to dispose of, this litter absorbs of odors naturally instead of masking it with scented oils or other synthetic scents!
Here's another good website on different kinds of natural litters: