Easy to Use Mice for Handicapped Persons?

Gavin Miller asks: "I've got a good friend with a disease that limits the use of his limbs, mainly his fingers. He says he can move a normal mouse but is no longer able to click. Does anyone know of input peripheral options that might be easier for him to use? I don't know exactly how much movement he does have in his fingers, or what he can or cannot do. I'm simply looking for some options that he can look into. Thanks!"