Calibre is a powerful and easy to use e-book manager
Thorium Reader
Thorium Reader is an easy to use EPUB reading application.
Sigil
Sigil is a multi-platform EPUB ebook editor .
MComix3
MComix is a user-friendly, customizable image viewer. It is specifically designed to handle comic books (both Western comics and manga) and supports a variety of container formats (including CBR, CBZ, CB7, CBT, LHA and PDF). MComix is a fork of Comix.
CoolReader
Cross platform open source e-book reader.
Foliate
A simple and modern eBook viewer
Bookworm
Read the books you love without having to worry about different format complexities like epub, pdf, mobi, cbr, etc.