Darktable
Darktable is a virtual lighttable and darkroom for photographers: it manages your digital negatives in a database and lets you view them through a zoomable lighttable....
TUTO : https://opensource.com/life/16/4/how-use-darktable-digital-darkroom
digiKam
digiKam is an advanced digital photo management application for linux/KDE, which makes importing and organizing digital photos a "snap". The photos are...
RawTherapee
RawTherapee is a free RAW converter and digital photo processing software. All changes can be applied and batch processed at a convenient time.
Fairly advanced control over RAW processing including demosaicing algorithm, gamma curves, etc. Supports its own profiles and presets.
Polarr
From advanced image filters to professional local adjustments, Polarr Photo Editor is the most versatile and powerful online photo editor in the web.
Shotwell
Shotwell is an open source photo organizer designed for the GNOME desktop environment. Features Import photos from folders or from any digital camera supported...
LightZone
LightZone is a digital photo editor software application. Its main purpose is to handle the workflow when handling images in various RAW formats.
Very easy to use tool, a bit limited, but a very well thought tool set, and how they work. Stil my most favourite tool (easy to use, does what I want, good results)
TUTO : https://opensource.com/life/16/6/how-get-started-lightzone
photivo
Photivo is a multi platform photo processor for RAW and bitmap images with 16 bit precision. Photivo tries to give the user as much control as possible to express his...
Corel AfterShot Pro
Corel AfterShot Pro is fast, flexible digital photography workflow software that combines powerful photo management, advanced non-destructive editing and complete RAW...
UFRaw
UFRaw (which stands for Unidentified Flying Raw) is an application which can read and manipulate photographs in raw image formats, as created by many digital cameras....
Free Open Source Mac Windows Linux GIMP