calibre
calibre is an e-book manager. It can view, convert and catalog e-books in all of the major e-book formats. It can also talk to e-book reader devices. It can go out to the internet and fetch metadata for your books. It can download newspapers and convert them into e-books for convenient reading. It is cross platform, running on Linux, Windows and OS X.
For more information, see the calibre About page
Screenshots
Usage
See the User Manual
Development
Setting up a development environment for calibre
A tarball of the source code for the current calibre release.