In 2000, Improbable Research sent a variety of regulation-violating items through the mail to see what would make it.
Post & Prejudice: [guardian.co.uk] "The Royal Mail is joining in the celebrations to mark the 200th anniversary of Pride and Prejudice with the release of a series of stamps featuring all six of Jane Austen's novels. Royal Mail commissioned the artwork by Angela Barrett." [Slideshow]
A Month of Letters is a challenge with two parts: mail something (anything!) every day the post runs in February and respond to every letter you get.
Tristan da Cunha has just been assigned its first postcode by the Royal Mail. This makes it easier for the inhabitants of these remote chunks of rock to receive mail. Easier, but still not easy - to get there, packages must first make their way to Cape Town and then travel 2,800 miles by fishing boat.
USPS advice on the Anthrax treat. You've read through the hype. Now read about what you really must know.