Cats make several trips a day to their litter boxes --and several more hours wearing the residue of the litter on their paws -- so the type of litter you use can be important to cat health. It is best to stick with simple clay litter. It's very inexpensive, runs about $3 for a 25 pound bag, and it does a great job absorbing excrement. There are all sorts of recyclable uses for it, and it breaks down easily in the environment.
Do not use litters with fragrances, they contain chemicals that cats may lick off paws when cleaning themselves, which can lead to toxicity and liver damage.
A litter made from recycled newspaper or one made from wheat straw are both environmentally friendly options. Buy litters that are sold in paper bags, not unrecyclable plastic tubes.