One of my uncles had a cat that lived with my grandmother (he went off to college at the time). This cat taught himself to use the toilet. At first, this cat seemed so dumb that the family figured he was mentally retarded. They decided to take him to the vet to be put to sleep to end his misery. After looking for him, they found him straddling the toilet seat and figured he had to be smart to figure that out. He used it regularly after that.
My cat just uses the litter box and the floor.