Undecimals
I have been a part of the Undecimals art exchange group for a few years. It is an art exchange through the mail, each year there is a group (or two groups) of twelve artists. Each artist is given a month and mails the other 11 an edition or multiple on their month, and receives something wonderful the other 11 months of the year. The Undecimals website has images of past projects and the Artist's Books and Multiples blog has posted about some of the projects.