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.
they're getting so popular now, that art companies and the sort are making a variety of holders just for these little mini arts. And you can find a bunch of magazines and books entirely regarding ATC's.
I treat em' as mini collages most of the time. Though occasionally I've just made them entirely a small painting. But really you could do anything. As long as it fits on a 2 1/2 by 3 1/2 card, go for it.