Traditional clay kitty litter is not very earth friendly as the clay is strip-mined from the Earth destroying animal habitats. Not only is clay litter bad for the planet, it can be dangerous for your cat as it contains silica dust which is a known carcinogen.
I have tried several alternative, earth-friendly kitty litter options - ground corncobs, wheat, recycled newspaper pellets. However my absolute favorite is pine pellets. Pine pellets are a by-product from sawmills in which sawdust that would normally end up in landfills is formed into pellets. The pellets have a fresh woody smell, are low tracking, and are very absorbent. I find that the pine pellets absorb odor better than clay litters and last longer. My cat made the switch to pine pellets very easily and since he loves to dig around in his litter I'm glad he is not exposed to the silica dust found in regular litter. I personally use Publix's Greenwise Pine Pellets which performs the same as brand name pine litters but is more reasonably priced.