Library Books is a free Mac OS 10.3 application for keeping track of the library books you have borrowed. At this time it works with libraries that use the SIRSI, Dynix or Spydus catalogue systems. The developer, Harold Chu, is dedicated to making it work with many libraries (for a list of supported libraries, see the above link). If your library is not listed, you might send him an e-mail and see if he can add it. If your library uses SIRSI, Dynix or Spydus, and you know the settings/URLs, it is possible you can use this by manually putting in the settings for your library.
It also ties into iCal, and will automatically remind you when books are due, along with a listing of just which books are due. It looks like it will allow you to renew them online, as well as browse the catalog.