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 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.

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.

List of websites that offer free ePub/mobi eBooks for download.