The Kindle reader is available for all operating platforms and it now supports the  .epub format.

.epub (electronic publication) is a widely used format and has been proposed as an international standard. The very best reader is Calibre. It can read and convert all formats (including word, PDF and others). It is however only available for Windows, Mac and Linux. For Android devices you may download a reader like Moon+ (ad-free) that can read most of the currently available formats, including .mobi.