If you're old enough to remember using cassette tapes, you might just get a kick out the Cassette Tape Bookmark.
As you might expect, the Cassette Tape Bookmark looks exactly like a miniaturized version of traditional cassette tapes that used be all the rage, before those newfangled CDs entered the picture. The miniaturized cassette folds open to reveal two magnets on the inside that allow you to securely clamp it onto the page. The tape part dangles down the length of the book and let's you easily find where you last left off.
Available in a pack of three, the Cassette Tape Bookmark comes in different colors, and even allows you to write on them, similar to how you would on traditional cassettes when you made a mixtape for a friend.