It was already mentioned, but I would have said "sugar glider". I'm not sure they are the smallest of mammal pets, but they are tiny. I took care of a couple of sugar gliders and they were so cute. Some places even sell pouches, called bonding bags, that a person could wear with the sugar glider riding inside the pouch. These marsupials are nocturnal, so a potential owner would have to aware of that. They also need a tall cage, like an aviary, so they could have room to jump and glide. This website has plenty of information about sugar gliders: http://www.sugarglider.com/
