VIPS is a free image processing system. It is good with large images (images larger than the amount of RAM in your machine), and for working with color. VIPS has been used and developed in the EU projects CRISATEL, VASARI, MARC, VISEUM, ACOHIR, ARTISTE and MUSA. The Department of Electronics and Computer Science at the University of Southampton use it as an image processing framework in student projects and PhD research. It's used at The National Gallery, London for much of their imaging research. VIPS is used in a number of other museums and galleries around the world, mostly for infra red reflectogram mosaic assembly.
One of the best graphics programs I have seen. It can view almost any format graphic file there is. And it can also convert graphics files from or to nearly any format. It has an excellent graphic browser and a wonderful slide show. Wonderfully written software, and intuitive to use.˙ Which is very fortunate, the documentation sucks.