LibraryThing has a discussion about weird objects used as bookmarks. For the most part these are discovered by librarians who deal with receiving and re-shelving borrowed books. I compiled a convenient list of such bookmarks. Just keep in mind that you will have to go through it in order to find out what the absolute strangest bookmarks are.
piece of bark
lock of hair
endorsed checks with a deposit slip
Abductees Anonymous member card
package of gum
outline for an unwritten book
chewed chewing gum