Clay cat litter (including many clumping types) is the most readily available, inexpensive and commonly used cat litter, so I know that many of you may still be using it. This doesn't mean you care about your cats any less, but it probably means that you don't know how bad it is for the environment and, potentially, your cat(s).
One major fact that isn't readily known is that clay cat litter is strip mined. Add to that, clay clumping litter does not biodegrade in landfills.
Clay litter contains silica (a known cancer causing agent when inhaled) and clumping clay litter contains sodium bentonite, which expands to 15 times its original mass and becomes completely insoluble. Think about what that does when your cat grooms itself and the sodium bentonite gets into their digestive system.
Here is the full blog post on the subject from moderncat.
More resources for further information: