RawTherapee - RawTherapee is a free RAW converter and digital photo processing software.
ACDSee - ACDSee is an image viewer that uses very little system resources, and it has a small, sleek size that won't take up much space on your system.
Darktable - darktable is an open source photography workflow application and raw developer.
Photoscape - high quality photo editing software, that enables you to fix and enhance photos.
digiKam - Professional Photo Management with the Power of Open Source
Polarr - MacOS, Windows, iOS, Android and online photo editing tools & free photo editors.