Calibre is a free and open source e-book library management application developed by users of e-books for users of e-books. It has a cornucopia of features divided into the following main categories:

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.
calibre can sort the books in your library by: Title, Author, Date added, Date published, Size, Rating, Series, etc.
In addition, it supports extra searchable metadata:
o Tags: A flexible system for categorizing your collection however you like
o Comments: A long form entry that you can use for book description, notes, reviews, etc.
You can easily search your book collection for a particular book. calibre supports searching any and all of the fields mentioned above. You can construct advanced search queries by clicking the helpful "Advanced search" button to the left of the search bar.
You can export arbitrary subsets of your collection to your hard disk arranged in a fully customizable folder structure.
Finally, calibre will even go out onto the internet to find book metadata based on existing title/author or ISBN information. It can download various types of metadata and covers for your books, automatically. The metadata system is written using plugins so that different types of metadata sources can be supported in the future.

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. Examples of supported news sites include:

o The New York Times
o The Wall Street Journal
o The Economist
o Time
o Newsweek
o The Guardian
o ESPN
o and many, many more…

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!