RawTherapee
RawTherapee is a free and open source, cross-platform raw image processing program
Darktable
Darktable – free and open source cross-platform software for processing and cataloging RAW images
XnConvert
XnConvert is a fast, powerful and free cross-platform batch image converter, It allows to automate editing for your photos
PhotoQt
PhotoQt is a free and open source multiplatform, fast and simple, yet powerful and highly configurable image viewer
Nomacs
Nomacs – free and open source multiple platforms fast image viewer with additional editing functionality
Fotoxx
Fotoxx is a free open source Linux program for image editing and collection management
Phototonic
Phototonic – free and open source software for viewing and organizing images with simple editing functions for Linux
XnView MP
XnView MP – free and cross-platform, versatile and powerful software for viewing, organizing, and editing photos
RPhoto
RPhoto is a lightweight free and open source software for easy photo handling on Linux and Windows
qView
qView – free and open source cross-platform full-featured minimalistic image viewer