The most valuable stamp in the world is a red smudge, but the coolest postal payments just might be these itty bitty stickies from 1970s Bhutan. They'd legally get your letter where it needed to go, and play the country's national anthem (yes, really!).
Bhutan—a somewhat isolated (and very happy) kingdom bordered by India and China along the Himalayas—might seem like an unlikely spot for philately innovation. But it's become a kind of hub for novelty stamps since starting to produce its own in the 1950s—and the backstory that sounds like some kind of bizarro Lost Horizon-style Shangri-La. The concept was hatched by an American entrepreneur and all-around swashbuckling guy named Burt Todd at the request of the Bhutanese royal family, who were hoping to give their nation a bit of an economic kick with these international calling cards.
After early efforts experimenting with "3D" effects (which look like lenticulars) and materials ranging from silk to gold foil, these fully-functional records—the "first talking stamps"—were created to play clips from the national anthem or a history of Bhutan (narrated by Todd himself, natch). WFMU has a couple clips (via Dangerous Minds) that you can listen to here and here.
Decades later, after Todd's death in 2006—tangential, but you really should read his obit in the New York Times—they released a few set with CD-ROM capabilities, which, like the vinyl, can still be found with a little searching on Ebay. No word on whether you need a Lilliputian phonograph to go along with it. Here's hoping the VR, Oculus Rift-enabled collection is coming soon. [Dangerous Minds; Bhutan Philately; Bhutan Postage Stamps; New York Times]