Image processing library (LGPL) and user-interface (GPL). VIPS is a free image processing system. It aims to be about half-way between Photoshop and Excel. It is very bad at retouching photographs, but very handy for the many other imaging tasks that programs like Photoshop get used for. It is good with large images (images larger than the amount of RAM you have available), with many CPUs (VIPS is also part of the PARSEC suite), for working with colour and for general research & development.
It comes in two main parts: libvips is the image-processing library and nip2 is the graphical user-interface. Both work on Unix (with convenient packages for most popular distributions), Windows (NT, 2k and XP tested) and Macintosh (OS 10.2 and later).