Sealand postally used cover
Sealand is a micronation off the coast of England, consisting of an enormous WWII gun and observation platform that was built off the coast by the British government and then abandoned after the war.
An enterprising person claimed it as his own and established his own country that isn't actually recognized by any other country. The original "prince" has since passed away but his sons carry on the arrangement.
They sell stamps and a few coins to collectors, but I wanted an actual postally used cover, so I wrote to the "government" of Sealand and got a reply. I believe the cover to be quite scarce, unlike the mint stamps and first day covers created for collectors.
The cover bears a Sealand stamp and cancelation, as well as a postmark from Royal Mail representing the payment of UK postage when the cover was placed into the British mail system. Since Sealand is not a recognized country its stamps aren't recognized, either, hence the necessity of paying an insufficient postage penalty to Royal Mail.