F. C. Schang collected the calling cards of prominent artists and musicians and in the late 20th century, donated a collection of them to Met Museum.
Calling cards derived from a custom, originating in England, in which messages were inscribed on the backs of playing cards. Cards made for the express purpose of sharing hand-written messages were manufactured beginning in the eighteenth century; by the early-nineteenth century, calling cards had become a popular means for sending well wishes, holiday greetings, condolences, and messages of courtship.
The cards include those of Klee, Renoir, Pissarro, Rodin, Monet, Mondrian, Braque, Toulouse-Lautrec, Manet, and many more. I think my favorites are Piet Mondrian's (above) and Joan Miró's, the former because it's very much in keeping with the artist's style and the latter because it isn't:
Schang published a book of these cards in 1983 — it's long out of print but you can get one here (signed, no less). He also collected the calling cards of generally famous people, singers, pianists, and violinists. (via greg allen)