Kchmviewer is a free, open-source CHM (MS HTML help) and EPUB viewer written in C++ for Unix, Mac and Windows systems. Unlike most existing CHM viewers for Unix, it uses Trolltech Qt widget library, and could optionally be compiled for better KDE integration.
save-as-ebook - Save a web page/selection as an eBook (.epub format) - a Chrome/Firefox/Opera Web Extension
Inspire your readers on their desktops, iPhones, Pads and Android Tablets! You can create very fast and efficiently an interactive pageflip, ePaper or flipbook from your own PDF with page2flip. Integrating videos, flash animations and links is also possibly.
Jomic is a viewer for comic books stored in CBZ, CBR, and PDF files. Features include a two page mode, caching and support for several image formats (PNG, JFIF/JPEG, TIFF, and GIF). It is Open Source software distributed under the GNU General Public License.
"Scriba eBook Maker" is a project for building ebooks (in ePub, PDF and zip formats) using contents already available in other formats (e.g., HTML, PDF, XML, etc.) both on the Web and on file systems.
GonVisor is a viewer of pictures, comics, magazines, books, images in electronic format. Created to view image sequences, fitting them to the proper size, and to create and open cbr/cbz/cba/zip/rar/ace files of images.
MComix is an user-friendly, customizable image viewer. It is specifically designed to handle comic books, but also serves as a generic viewer. It reads images in ZIP, RAR, 7Zip or tar archives as well as plain image files.
Read the books you love without having to worry about different format complexities like epub, pdf, mobi, cbr, etc.
Cool Reader is fast and small cross-platform XML/CSS based eBook reader for desktops and handheld devices. Understands a number of text document formats, suppurt OPDS (in new ui-opengl version). Fully customizable palette, text reformatting with any font size, font...
Calibre2opds can generate OPDS and HTML catalogs from the metadata of the 
calibre e-book library manager. These catalogs can then be used both to browse your e-book library online and also to download books on demand.
Calibre-Web is a web app providing a clean interface for browsing, reading and downloading eBooks using an existing calibre database.
Free e-book manager offering library management, multi-format conversion, syncing to devices, news download as e-books, built-in viewer, editor, and privacy.
HakuNeko is a cross-platform downloader for manga and anime from various websites. HakuNeko was made to help users who download media for circumstances that requires offline usage. The philosophy is ad-hoc consumption, get it when you want to read/watch it.
Thorium Reader is a free, open source multi-language EPUB reader for Windows 10, MacOS and Linux. It's privacy aware and highly accessible for visually impaired and dyslexic people.
Koodo Reader is an all-in-one e-book reader that can help you better manage and study your e-books. It's free and open-source.
An ebook reader application supporting PDF, DjVu, EPUB, FB2 and many more formats, running on Kindle, Kobo, PocketBook, Linux and Android devices.
OpenComic is a free and open-source comic and manga reader for Windows, Mac and Linux, written with Node.js and using Electron.
