Sumatra PDF
Sumatra PDF supports viewing ePub ebooks. Sumatra is free, open-source and supports many other formats, like PDF, mobi, XPS, DjVu, CBR, CBZ.
FBReader
FBReader is a cross-platform, open-source eBook reader that reads ePub files.
Cool Reader
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
Calibre is an eBook library manager with built-in eBook viewer.
Callibre can also convert between various eBook formats.
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