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.

You might also be interested in Lex Mercatoria. It's a system of common law used between merchants in medievil Europe for dispute resolution, particularly in a time when the largest truly effective government was at the level of the nation-state: http://en.wikipedia.org/wiki/Lex_mercatoria

I really wish I had more time and energy to devote to the study of polycentric law. It's something I feel very strongly about, and where I know enough to realize my understanding is incomplete :)

As I see it, the western legal system which relies on written analytical laws appears indeed as an "alternative", but western judges do for example make use of judicial precedents too. This system is based on a set of principles that have just a lesser use in western legal system.