From the About page:

calibre is a powerful and easy to use e-book manager. Users say it’s outstanding and a must-have. It’ll allow you to do nearly everything and it takes things a step beyond normal e-book software. It’s also completely free and open source and great for both casual users and computer experts.

History:

calibre started life on 31 October, 2006, soon after the release of the SONY PRS-500, the first e-ink based reader to be sold commercially in the US...The PRS-500 did not work at all with Linux...so I decided to reverse engineer the USB protocol that it used...and calibre was born, albeit named libprs500

Today calibre is a vibrant open-source community with half a dozen developers and many, many testers and bug reporters. It is used in over 200 countries and has been translated into a dozen different languages by volunteers. calibre has become a comprehensive tool for the management of digital texts, allowing you to do whatever you could possibly imagine with your e-book library. Reading is very important to me and one of my goals has always been to prevent either the fragmentation or the monopolization of the e-book market by entities that care solely for short-term goals.