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Sumatra PDF supports viewing ePub ebooks. Sumatra is free, open-source and supports many other formats, like PDF, mobi, XPS, DjVu, CBR, CBZ.
FBReader is a cross-platform, open-source eBook reader that reads ePub files.
Cool Reader supports ePub and a few other formats.
EPUBReader Chrome aFireFox extension
EPUBReader is a FireFox and Chrome web browser extension.
Very light on features but does the basic job of showing ePub files.
Calibre is an eBook library manager with built-in eBook viewer.
Callibre can also convert between various eBook formats.
Details of ePub format
ePub is an open format defined by the Open eBook Forum of the International Digital Publishing Forum.
ePub format is a based on XHTML, XML and CSS.
ePub files usually have .epub extension.
Free ePub books
List of websites that offer free ePub/mobi eBooks for download.