Wikipedia
Kitabu
Kitabu is a free EPUB reading application for Mac OS X. It is easy to use and has a low memory imprint. The app has library, shows the table of contents and supports both ePub2 and ePub3 formats.
Kitabu was released to the public on 20 February 2012 and it has been downloaded over 280,000 times.
Since the official website is defunct since sometime after 14 May 2016, its future development is currently unknown.