Do you know ATC cards ?
Artist trading cards (or ATCs for short) are small, baseball-card sized works of art that can be traded with other artists as a way to get creative, connect with other artists, make contacts, and collect great art!
I love that idee, and make and collect myself.
Some cards is called ACEO, ACEO stands for Art Card Originals and Editions.
They are cousins of ATC's , both are by definition cards of artwork measuring
2.5 x 3.5 inches, make ACEO cards as well.
here is some sambles:
its really fun to have a great art collection in a shoebox :)