Art Card, Editions, and Originals.
Also known as the ATC - artist trading card.
Art small enough to be mailed. Small enough to be traded. Apparently, according to Wiki- they started in the 90's in switzerland. So this little idea isn't that old. And I must say I've spent a lot of time admiring other artist's ATC's, but furnishing none of my own.
But now I've finally started, and I posted some on etsy for trading or selling.