GIMP is the GNU Image Manipulation Program. It is a freely distributed piece of software suitable for such tasks as photo retouching, image composition and image authoring. It can be used as a simple paint program, an expert quality photo retouching program, an online batch processing system, a mass production image renderer, a image format converter, etc.
LibRaw is a library for reading RAW files obtained from digital photo cameras (CRW/CR2, NEF, RAF, DNG, and others). LibRaw is based on the source codes of the dcraw utility, though some of the drawbacks of dcraw have been eliminated. The users of the library are provided with an API to be used when writing their software programs.
Image Alchemy has the ability to convert between over 90 different raster image formats (including all colorspace and compression variations of each format). Color management features include gamma correction, undercolour removal, seven high quality dithers, colorspace conversion, and palette manipulation. It also includes a Postscript interpreter.
LibUFO is a C++ core library for graphical user interfaces (GUI). It is mainly used as OpenGL GUI toolkit. Most standard components (such as buttons, labels, menus, and internal frames) are included. It is customizable via look-and-feel and themes. There are three built-in video drivers (respectively using GLX/X11, WGL/Win32, and LibSDL) which are selected and loaded at run-time. You may also create your own backend or use a dummy driver to include libUFO widgets into an already existing OpenGL context.