Yomikiru is an offline reader for already downloaded comics, manga, webtoons and epub. Supports continuous vertical scrolling mode with no gap in between images. Supports drag and drop of files.
OpenComic - Open-source comic and manga reader for the desktop.
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IrfanView - IrfanView ... one of the most popular viewers worldwide.
Mcomix - MComix is an user-friendly, customizable image viewer.