Library Books connects over the web to a public library catalogue and retrieves the patron's data -- items checked out, items ready for pick up, items requested but not available yet. It lives in the menu bar, where it provides visual feedback to alert for overdue items or items ready for pick up; the due date information is also shared with iCal.

It is unobtrusive, well crafted, and it works almost flawlessly for me; the developer is responsive and listens well. It's not, perhaps, an application I couldn't live without; but it's so convenient that, although it's free, I sent the developer a small donation.