Okular
Open Source
Okular is a universal document viewer based developed by KDE.
Polar
Open Source
Polar is an integrated reading environment to actively read, annotate, create flashcards, and track progress.
EPUBReader
Free
EPUB is an e-book file format with the extension .epub that can be downloaded and read on devices like smartphones, tablets, computers, or e-readers.
FBReader
Open Source
FBReader is a free ebook reader. Main ebook formats: ePub (including main features of ePub3), Kindle azw3 (mobipocket), fb2(.zip).
Cool Reader
Open Source
Cool Reader is a configurable eBook viewer.
Compare calibre with alternatives
Compare Cool Reader and calibre and decide which is most suitable for you.