I've come up with a rather simple idea to use the wonderful Mac program Delicious Library to help me track library books I've checked out and help avoid (or at least reduce) late fees.
Delicious Monster allows you to track books, video games, music, and movies you have loaned out to people. In the Apple Address Book, I have an entry for myself. When I check a book out from the library, I add it to my Delicious Library. I then loan the book out to myself with a due date of, you guessed it, the due date.
Due items show up in two places: Delicious Library and iCal.
This trick should be usable on just about any similar program that has the ability to track things that have been loaned out.
It would be nice if Delicious Library 2.0 would have the built-in ability to track books borrowed as well as book lent out.