Xeer is one example of the success of private, customary and polycentric law. The Lex Mercatoria as the other poster noted is one more example.

See also Zomia: http://opinionator.blogs.nytimes.com/2012/02/14/the-undiscov...

Medieval Iceland: http://mises.org/daily/1121

and Ireland (page 3): https://mises.org/journals/lf/1971/1971_04.pdf

People are waxing about the superiority of "British common law" while its roots are a system of the Anglo-Saxons extremely similar to the Xeer.