I am a total Calibre addict, though my extremely limited knowledge of CSS does not allow me to use all of its potential. To all the uses that have been mentioned already, I would also add that you can use Calibre as a server and "braodcast" your library, and connect to it wirelessly with your device to pick up books if for whatever reason you are unable to connect it via USB cable. It saved me when a firmware update on my device made it unrecognisable to my laptop!
Viva Kovid and his bunch!
