The EPUB format is rapidly gaining popularity and as of 2011 is the most widely supported vendor-independent XML-based e-book format. The format can be read at least by the Kobo eReader, Blackberry Playbook, Apple's iBooks app running on iOS devices such as the iPhone, iPod Touch and iPad, Barnes and Noble Nook, Sony Reader, BeBook, Bookeen Cybook Gen3 (with firmware v. 2 and up), COOL-ER, Adobe Digital Editions, Lexcycle Stanza, BookGlutton, AZARDI, FBReader, Aldiko, Moon+ Reader and WordPlayer on Android, Freda on Windows Mobile and Windows Phone 7, the Mozilla Firefox add-on EPUBReader, and Okular. Several other desktop reader software programs are currently implementing support for the format, such as dotReader, Mobipocket, uBook. [Source: Wikipedia]
