Have you seen the Moo Cards on Flickr? I love them. They're like miniature business cards. For $19.99 you get a hundred little cards, approximately 1" x 3.5". Lots of artists have been getting them to trade. I never seem to be able to commit to a design, though.
Now Susie LaFond is hosting an "extreme inchies" swap for Moo sized cards that are created as original collages. It's a great format. I made several this morning. This is my favorite.
If you haven't tried this format, you really should consider it! It's lots of fun to search through your studio for tiny images and doodahs. This image is from Paper Whimsy.