ePub is an electronic book format that has become the industry standard, allowing eBooks that use this format to be read on a wide variety of eReaders, from dedicated hardware to desktop software to online based readers. Here are a few examples;

Other than hardware devices, you can also read .epub books on your desktop or mobile phone using free e-reader software like Calibre, Lucifox, FB Reader, and Stanza.
 
Free ePub Reader for Mac - Calibre
Calibre is a one-stop solution to all your e-book needs. It is free, open source and cross-platform in design and works well on OS X and. Calibre is meant to be a complete e-library solution and thus includes library management, format conversion, news feeds to ebook conversion, as well as e-book reader sync features and an integrated e-book viewer.

Calibre can be used as an ebook viewer for all major DRM-free ebook formats, and it can convert from many file types, including CBZ, CBR, CBC, CHM, EPUB, FB2, HTML, LIT, LRF, MOBI, ODT, PDF, PRC, PDB, PML, RB, RTF, TCR, TXT.

Free ePub Reader for Mac - Lucifox
Lucifox is an e-book reader extension for the Mozilla Firefox web browser. It enables you to read e-books and browse e-book catalogs in a browser window. It can also be used to create e-books from web feeds. It supports e-books in the EPUB format and catalogs of the OPDS kind.

Free ePub Reader for Mac - FB Reader
FB Reader opened all files without errors, but displayed some images and headings differently than the HTML said it should be handled. Also, lack of support for the NCX table of contents specified in the ePub standard makes navigating books more difficult than it could be.

Free ePub Reader for Mac - Stanza
Stanza is the most popular electronic book reader for a reason. Intuitive ease of use, unmatched customizability, and powerful library organization features all blend together to make finding, obtaining, and reading your books a joy.