The simple and versatile Sumatra PDF has been gaining useful features with every new version. It can now be used as an ebook reader too. It can handle the EPUB and Mobi formats.
After comparing it with the Calibre ebook reader, I actually prefer Sumatra PDF for reading ebooks. The Calibre sets of tools are many and heavy. Its ebook reader has a crappy interface and the display is not that great either. In contrast, Sumatra PDF gives a dead-simple interface that put the content upfront and all one needs to do is flip to the next or previous page.
Tried with: Sumatra PDF 2.1.1