nomacs is a free, open source image viewer, which supports multiple platforms. You can use it for viewing all common image formats including RAW and psd images.
Ranked in these QuestionsQuestion Ranking
Pro Detailed view of image metadata
Pro You can work with hot keys that can be reassigned as you want them
Pro Frameless view
Pro Allows to cut (frame) images
It is important if you have to work with photos of documents and want to cut off the background on which you made photos.
Pro Highly customizable
Pro Comparing two images
This feature is called Synchronization. With the synchronization it is possible that multiple viewers perform the same action (like panning, zooming, etc.). This feature is useful when comparing two images. More at http://nomacs.org/synchronization/
Pro Lots of Image Adjustment Tools
Con Recent version has slowed down
The latest version (3.8) suddenly became very slow in Fedora 26. Slow to step from image to image; slow to open a directory; slow slow slow. Downgrading to the previous version (3.6) restored speed.