Darktable - darktable is an open source photography workflow application and raw developer.
RawTherapee - RawTherapee is a free RAW converter and digital photo processing software.
UFRaw - The Unidentified Flying Raw conversion utility
Filmulator - A simple raw photo editor based on the process of developing film.
LightZone - LightZone is a digital photo editor software application.
Darkr - This isn't your normal filter app. The hint's right there in the icon. Instead, Darkr tries to recreate the process of using a real-life darkroom to develop your photos. For starters, your camera roll appears as a bin of negatives....