Image Viewers
For many, tools to easily view images in their best quality are just as important, if not more important, than the tools needed to create and edit them. Mac users have the ubiquitous built-in Preview, and recent Windows users have the built-in Photos app, but there are several options other than them, such as HoneyView.
IrfanView
Fast and compact image viewer/converter trying to be simple for beginners and powerful for professionals. Many supported file formats and features.
XnView
XnView is a utility for viewing and converting graphic files. XnView is provided as FREEWARE for private non-commercial or educational use (including non-profit organization).
FastStone Image Viewer
FastStone Image Viewer is a fast, stable, user-friendly image browser, converter and editor.
XnView MP
XnViewMP is a free software to view, organise, convert graphics and photos files or to create slide show, contact sheet, HTML pages.
ACDSee Photo Studio
ACDSee becomes ACDSee Photo Studio — ACDSee Photo Studio Standard 2018 continues the ACDSee legacy.
nomacs
nomacs is a free image viewer for windows, linux, and mac systems, which is licensed under the GNU Public License v3.
JPEGView
JPEGView is a lean, fast and highly configurable viewer/editor for JPEG, BMP, PNG, WEBP, TGA, GIF and TIFF images with a minimal GUI.
PDF Reader
PDF Reader is the one app you can rely on when you need a portable solution to work with PDFs.
Preview
Preview is a simple and elegant application by Apple for viewing and working with images and PDF documents.
ComicRack
Discontinued
No updates since 2016. The dev, cYo has been MIA since 2017 and forum is full of spam bots.
It is an all-in-one solution to read and manage your eComic library. You can think of it as ITunes for eComics with much of the same functionality.
HoneyView
HoneyView is a fast image viewer that supports archived image file for comics/manga/photos.Its design is light and the interface skin can be changed.
Comix
Discontinued
The program is no longer updated; this is the reason the 
Mcomix fork was started. Last version of Comix, 4.0.4, released in April 2009, can be still downloaded from the official website.
Comix is an image viewer specifically designed to handle comic books. It reads ZIP, RAR and tar archives (also gzip or bzip2 compressed), as well as plain image files.
Gwenview
Gwenview is a fast and easy to use image viewer by KDE, ideal for browsing and displaying a collection of images.
Zoner Photo Studio X
Your Lightroom® and Photoshop® alternativeZPS X is the most universal photo software there is. RAW processing. Layer support. Presets. Retouching tools.
Xee
Xee is a lightweight, fast and convenient image viewer and browser. It is designed to be a serious tool for image viewing and management, with a sleek and powerful interface.
Eye of GNOME
Eye of GNOME is the official image viewer for the GNOME Desktop environment. With it, you can view single image files, as well as large image collections.
Geeqie Image Viewer
Geeqie is a lightweight Gtk+ based image viewer for Unix like operating systems. It was forked from GQview.
YACReader
YACReader (Yet Another Comic Reader) is a free cross-platform comic reader with support for multiple comic files and image formats.
iNFekt NFO Viewer
A text viewer application that has been carefully designed around its main task: viewing and presenting NFO files.
Mylio
Easily gather, organize and protect all of your photos across all your devices. Use Mylio to rediscover, enjoy, and share your memories—anytime, anywhere.
Mcomix
Discontinued
The last version is from 2016. See https://sourceforge.net/projects/mcomix/files/
MComix is an user-friendly, customizable image viewer. It is specifically designed to handle comic books, but also serves as a generic viewer.
Visio Viewer
Visio Viewer opens Visio drawings directly on a computer without conversion on external Web Servers. Viewing Visio documents is as simple as drag and dropping of files (with a .vsd, .vsdx, .vdx extension) to the Application window.