XnView MP stands for Multi Platform = it is available for Windows, Linux and MacOS. It is a continuation of the older XnView Classic. In my opinion it is a great alternative to Windows photos app or any Linux image viewer that comes preinstalled. I even think it is an alternative to IrfanView. It is really fast and simple to use, supports more than 500 image formats, has built in folder browser, image converter, editor... AND MORE!
RawTherapee - RawTherapee is a free RAW converter and digital photo processing software.
IrfanView - IrfanView ... one of the most popular viewers worldwide.
Darktable - darktable is an open source photography workflow application and raw developer.
FastStone Image Viewer - FastStone Image Viewer is a fast, stable, user-friendly image browser, converter and editor.
digiKam - Professional Photo Management with the Power of Open Source
JPEGView - JPEGView is a small and fast viewer/editor for JPEG, BMP, PNG, GIF and TIFF images.