Murasaki is an EPUB reader application that has a simple user interface and various features.
The app is a scroll-based reader like usual web browsers, not a page flip-based reader like iBooks and can open different EPUBs in multiple windows.
Also the app features popover (you can check a related page, footnote, and so on quickly without moving page), image panel (you can read through text with reference to these images), searching (you can search a word that is contained in a EPUB), bookmarking (you can bookmark pages, and can refer to them later), and much more.
Note: can not open DRM-encrypted EPUBs)