The Hanseatic League, as described in The Anglosphere Challenge
(p 132-133), by James C. Bennett, sounds like a libertarian society. It existed for 500 years in the Middle Ages between the 13th and 17th centuries and "was a coalition of the willing. It never required unanimity for action, nor did it act by majority vote. Those parties that felt a need to do something consulted each other and upon reaching consensus, proceeded to execute the decision, while those who remained outside the consensus disassociated themselves from it. ... The only common institution was the Hanseatic Diet, which was stirctly a forum for mutual discussion and the formation of suballiances to accomplish specific tasks."
From the wikipedia entry
, the League sounds less like an organization organized along libertarian lines, but the entry seems more descriptive of the bare facts rather than a discussion of its structure and implications that follow, as is true in Bennett's book.