The Cambridge Libraries Widget is available via a Quick Link called "Library Account & Renewals", located on the right-hand side menu of the Balfour Library homepage.
You can check what books you have on loan, and request their renewal. You can also do a quick search of the catalogue and make reservations.
Just log in with your library card bardcode number and your surname, and the rest should be straight forward - though do please ask library staff for help if not.
If you use iGoogle, or Facebook, or CamTools, you can load this "widget" into those pages - see this site for information and instructions: http://www.lib.cam.ac.uk/toolbox/camlibwidget.html