Calibre is a one stop solution to all your e-book needs. It is free, open source and cross-platform in design and works well on Linux, OS X and Windows. calibre is meant to be a complete e-library solution and thus includes library management, format conversion, news feeds to ebook conversion, as well as e-book reader sync features and an integrated e-book viewer.

Library Management
calibre manages your e-book collection for you. It is designed around the concept of the logical book, i.e., a single entry in your library that may correspond to actual e-book files in several formats.

E-book conversion
calibre can convert from a huge number of formats to a huge number of formats. It supports all the major e-book formats. The full list of formats can be found here.
The conversion engine has lots of powerful features. It can rescale all font sizes, ensuring the output e-book is readable no matter what font sizes the input document uses. It can automatically detect/create book structure, like chapters and Table of Contents. It can insert the book metadata into a "Book Jacket" at the start of the book.

Syncing to e-book reader devices
calibre has a modular device driver design that makes adding support for different e-reader devices easy. At the moment, it has support for a large number of devices, the complete list of which is here. Syncing supports updating metadata on the device from metadata in the library and creation of collections on the device based on the tags defined in the library. If a book has more than one format available, calibre automatically chooses the best format when uploading to the device. If none of the formats is suitable, calibre will automatically convert the e-book to a format suitable for the device before sending it.

Downloading news from the web and converting it into e-book form
calibre can automatically fetch news from websites or RSS feeds, format the news into a ebook and upload to a connected device. The ebooks include the full versions of the articles, not just the summaries. calibre has over three hundred news sources and the news system is plugin based, allowing users to easily create and contribute new sources to calibre. As a result the collection of news sources keeps on growing!

Comprehensive e-book viewer
calibre has a built-in ebook viewer that can display all the major ebook formats. It has full support for Table of Contents, bookmarks, CSS, a reference mode, printing, searching, copying, customizing the rendering via a user style sheet, embedded fonts, etc.

Content server for online access to your book collection
calibre has a built-in web server that allows you to access your ebook collection using a simple browser from any computer anywhere in the world. It can also email your books and downloaded news to you automatically. It has support for mobile devices, so you can browse your collection and download books from your smartphone, Kindle, etc.