Darktable

Darktable is a free, open-source photography workflow application and RAW developer. It operates as a non-destructive editor with powerful organizational capabilities and a comprehensive set of modules for color correction, tonal adjustments, and creative effects. Its strength lies in its scientific approach to image processing and highly configurable module system. Targeted at technically-minded photographers and those avoiding proprietary software, it is completely free.

It scores well for its professional-grade RAW processing capabilities, active development, and commitment to a free, open workflow. It is a top-tier free alternative to Lightroom, especially for Linux users.