RawTherapee - RawTherapee is a free RAW converter and digital photo processing software.
PixBuilder - Free Photo Editing Software for PC
Darktable - darktable is an open source photography workflow application and raw developer.
showFoto - ShowFoto is the standalone image editor of the digiKam project.
digiKam - Professional Photo Management with the Power of Open Source
Fotografix - In addition to being totally portable, Fotografix has a ridiculously small footprint.