The Pygmy Marmoset (Cebuella spp.) , as the name suggests, is an extremely small monkey. In fact, it's the smallest of all New World primates, the average adult weighing in at four ounces and measuring about 15 inches, almost half of which is tail.
They live in the rainforest, and are found in western Brazil, southeast Colombia, eastern Ecuador, and eastern Peru. Most of the Pygmy's day is spent in the forest canopy, where they feed by boring tiny holes in the tree bark with their teeth, and then sucking gum from the tree. They supplement this nutritious diet with spiders and other small insects, and the occassional fruit.
Pygmy Marmosets are monogamous, and live in extended family groups, (sometimes as large as 15), dominated by one mature male and female. The dominant female maintains her status by secreting hormones that suppresses the ovulation of the other females in the group. Despite this, all group members participate in the rearing of children, which are about the size of a human thumb when born, and so cute they can make your eyes bleed.
Pictures of (awww) baby Pygmies can be found at - http://www.rarespecies.org/pmarmo.htm
Sources -
http://www.honoluluzoo.org/pygmy_marmoset.htm
http://www.rarespecies.org/pmarmo2.htm
The 1982 Guinness Book Of Records