Library Elf! It will, if your library is supported on its system, remind you that your library books are due! You can set the reminders for a week before, three days before, etc. You can also set it to remind you of overdues on a regular basis (for me, daily!) as well as books on hold for you.
This is beyond fabulous for me. I have probably paid enough in library fines in my life to build an entire wing onto the building. Just ask my mother!