Darktable - darktable is an open source photography workflow application and raw developer.
digiKam - Professional Photo Management with the Power of Open Source
RawTherapee - RawTherapee is a free RAW converter and digital photo processing software.
XnView MP - XnView is a free software that allows you to view, resize and edit your images. It supports more than 500 different formats!
Affinity Photo - Affinity is the imaging and design suite for creative professionals exclusively for Mac.
FastStone Image Viewer - FastStone Image Viewer is a fast, stable, user-friendly image browser, converter and editor.