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/p/photo-editing.html - detailed article - Gimp, Paint.Net, Zoner Free, RawTherapie and many more.