And don’t forget this guy.Can the extra digits be of use, depending on where they're positioned? I would think that with proper technique, that could be a boon for typing or piano playing, if the right kind of duplication happens.
It's pretty rare for the extra fingers to be fully functional, but I suppose it could come in handy with extra training, like the genetically engineered pianist with twelve fingers in Gattaca.

(Actually, in the most common form of polydactylism, the extra digits are soft tissue without bones or tendons or any dexterity whatsoever.)