RawTherapee - RawTherapee is a free RAW converter and digital photo processing software.
Darktable - darktable is an open source photography workflow application and raw developer.
digiKam - Professional Photo Management with the Power of Open Source
Affinity Photo - Affinity is the imaging and design suite for creative professionals exclusively for Mac.
Polarr - MacOS, Windows, iOS, Android and online photo editing tools & free photo editors.
GIMP - GIMP is a multiplatform photo manipulation tool.