Sealand has been mentioned from time to time, usually in the micronations threads.
Basically a World War II air-defense fort located about 6 miles off the Suffolk coast, it was vacated by British forces, and later occupied by Major Paddy Roy Bates. Bates claimed that the fort was beyond the territorial jurisdiction of the UK, and declared an independent nation of Sealand. Coins and stamps followed.
The first issues, from 1969, depicted famous explorers. Here is a sheet showing the first issues:

Where there are stamps, you might expect covers, and I recently came into a handful of them, including this one, depicting the 1970 series of ship stamps on the front:

and a $1 miniature sheet on the reverse:

"Registered" for good measure, and yes, the contents are there. Can't be opened without disturbing the miniature sheet, so one is left to wonder what spectacular information might be contained within.
(Actually, I believe it is literature advertising new stamp issues, but who can say, for sure?)
Sealand was briefly in the news several years ago when a fire forced the evacuation of the inhabitant, and it is briefly in the news again as a possible site for Wikileaks' servers. Supposedly it was a data haven in years past, and with all the hubbub surrounding SOPA, PIPA and ACTA, perhaps they could actually make a go of that.
Wikipedia says an online casino is forthcoming.