Reader seeasea was kind enough to send me links to many an interesting Wikipedia article, including one about Sealand.
Although not recognized as a sovereign nation by the UN, Sealand is categorized as a micronation. Built as a sea fort during World War II, it was taken over by Paddy Roy Bates in 1967, over ten years after British Navy personnel had abandoned it. Since it is six miles off the coast of England, it is in international waters and out of the country's territorial water claim (at the time). Sealand had its own constitution, flag, currency, and passports-- it even had a war! So much better than a lame tree fort.