From Kovid Goyal
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.
I come from the Unix world, which means calibre is architected in little pieces each with its own command line interface which means that each piece of functionality is reusable not just in python programs but in any software. In fact adding complete support for converting any new ebook format to calibre requires the writing of only two converters format->html and html->format as well as a metadata reading/writing tool. All the other features of calibre will work automatically with these three converters in place. Similarly, writing a device driver requires very little work. You basically have to implement a well-defined interface (interface.py). Once you do that, your device will have full support in calibre.
Adding support for new Input/Output formats
Calibre is written primarily in Python with a few C extension modules. So you have to contribute plugins in either Python or C/C++.
A great feature is "Get Books" which allows you to search for a book by title and author and returns the list of web stores that sell it in ebook form, allowing you to easily find the lowest prices for popular books or search many different places for hard to find ebook editions.
The last year has also seen the maturing of the calibre plugin infrastructure, which has led to a vibrant community of user developed plugins that extend calibre's functionality in many novel ways. See the Index of plugins.
calibre has an all new metadata download system that allows you much more control over the process. For example, you can now choose a cover from many different online sources.
As always, calibre has gained support for dozens of new devices, conversion of a few new formats, notably, SNB and the Plucker formats. The performance of calibre has been greatly improved, especially with large libraries of tens of thousands of books.
While not directly calibre related, a new website that catalogues unrestricted access ebooks for sale was started: Open Books. All the books on this website are for sale, but they do not have DRM, which means you actually own them after paying for them. You can convert them to different formats, fix typos in them, view them on as many devices as you like. At calibre, we believe that DRM free is the future of digital information and Open Books is a small effort to help speed that future along.