When I was at school the library worked on a system of trust that meant you took the card out of the back filled in that you had taken it and then made sure you came back with the book before the deadline. Saw a posting about this library card generator on Stephen Lang's blog and he also mentions there is something about it on Brandon's blog The After-Dinner Payback. It's all bit of fun and really easy to do at the card-generator site.
