Darktable 1.6 - photo editor and manager

Darktable is a free and open-source photography workflow application and raw developer similar to Adobe Lightroom. A virtual lighttable and darkroom for photographers. It manages your digital negatives in a database, lets you view them through a zoomable lighttable and enables you to develop raw images and enhance them.
The new version 1.6 comes with a new Slideshow mode and some other interesting changes:

• support for HiDPI/Retina display, plus there's an OSX build again;
• support for arbitrarily sized images, 26770x13385 large TIFFs successfully tested;
• TIFF support completely rewritten, reads and writes 32bit float files now;
• JPEG files are exported with user-defined PPI value now (defaults to 300);
• color space conversions got faster thank to using OpenMP via LittleCMS;
• new defringe image processing filter added;
• the levels and lens correct plugins got an auto mode;
• linear Rec2020 is now available as a built-in profile.

Download link (MacOs, Linux): http://www.darktable.org/install/

Other free alternatives (Windows, MacOs, Linux):

• http://open-source-spot.blogspot.ro/2014/11/rawtherapee-42-free-image-processing.html
•  http://open-source-spot.blogspot.ro/p/photo-editing.html  - detailed article - Gimp, Paint.Net, Zoner Free, RawTherapie and many more.