Calibre

Description
calibre is a free and open source, primarily based in Python, and a cross-platform e-book library management application. The website boasts a number of features including: library management, e-book conversion, syncing to e-book reader devices, the ability to download news from the web and converting it into e-book form, a comprehensive e-book viewer, content server for online access to e-book collection, and an e-book editor for major e-book formats.

Platform
calibre runs on Linux, Mac OS X, and Linux.

Documentation and User Support
The webpage links to a user manual, available here. Their support also includes a list of Frequently Asked Questions, helpful tips and tricks in their blog, and a forum among other resources.

User Experiences
Macworld: Review

Dear Author Blog: Review

Softpedia: Review

Development Activity
Calibre Changes - http://bazaar.launchpad.net/~kovid/calibre/trunk/changes

Developer Blog for Major Changes - http://calibre-ebook.com/whats-new