This article will concentrate mostly on polydactyly in humans; for a discussion of polydactyly in other animals, see the bottom of the article.
In humans, it is usual to have five digits (four fingers and one thumb) on each hand, and five digits (toes) on each foot. Polydactyls have six or maybe even more digits on either their hands or feet, or both. The extra digits vary from small pieces of soft tissue to apparently complete digits. In Western societies, they are usually surgically removed during early life.
The condition is reported in about 2 children in every thousand (http://www.drgreene.com/21_182.html), although the frequency varies greatly from population to population. It is higher in some groups (e.g. the Pennsylvania Dutch in the United States) due to the founder effect[?] (http://scidiv.bcc.ctc.edu/rkr/Biology101/lectures/EvolutionMechanisms.html).
Historically, polydactyly has in some areas been considered an indication of witchcraft.
There are several types of polydactyly, of varying frequency. Here are some of them:-
A story that Marilyn Monroe was born with six toes resulted from the publication of photographs taken by the photographer Joseph Jasgur[?] in March 1946. The pictures were published in the book The Birth of Marilyn: The Lost Photographs of Norma Jeane (1991) by Jasgur and Jeannie Sakol[?]. Two of the pictures can be interpreted as showing six toes, although they can also be explained as tricks of the light. Since there is no corroborating evidence from other photographs or written records, the story is commonly dismissed as an urban legend (See Snopes (http://www.snopes.com/movies/actors/mmtoes.htm) and see  (http://marilynmonroepages.com/6toes.html) for photographic proof that her feet were normal).
Cats usually have four digits on the hind paws, and five on the front ones. Polydactyl cats usually have only one or two extra toes on each front paw, but can have as many as double the usual number, giving the appearance of having "double paws" (http://www.straightdope.com/mailbag/m6toecat.html).