I'm pretty guilty of using whatever is lying around as a bookmark. Usually it's the check-out stub for the library books. Or the odd grocery receipt.
But more interesting things have been used! Based on this ongoing discussion over on LibraryThing, breakfast items seem to be big among readers! Some of the strangest things used as bookmarks include:
(Bacon: credit, Pancake: credit, Marijuana: credit)
So, what have you used lately?