Darktable - darktable is an open source photography workflow application and raw developer.

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.

RawTherapee - RawTherapee is a free RAW converter and digital photo processing software.

Photoscape - high quality photo editing software, that enables you to fix and enhance photos.

Affinity Photo - Affinity is the imaging and design suite for creative professionals exclusively for Mac.

Fotor - Fotor is an all in one visual platform. It includes photo editor, collage maker, and graphic designer. You are free to make a visual look more beautiful.