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.
XPaint is an easy-to-use image editor which supports many standard and less standard paint procedures that demanding non-professional users would expect. It includes several advanced features like image processing algorithms, programmable C scripts, simultaneous editing of multiple images, and supports a wide variety of image formats.
SImg is a software for astronomical image processing. It contains a tool for visualization of highly dynamic images. The package consists of a library and applications which use it. Some of the features are deconvolution (various methods), Video4Linux support, PSF computation, approximation by linear combination of images and polynoms, various color transforms, automatic shifting of image sets (for addition and color composition), and more.
MultiSlice RTP is a radiotherapy treatment planning and medical image processing system developed for Silicon Graphics MIPS architecture systems and the IRIX operating system (5.3 and above). It allows you to manipulate upto 20 2D CT image slices as one 3D slice and run bulk batch processing algorithms on the images loaded. Written in C and linked against the MOTIF toolkit, it also runs on Xfree/Linux/Lesstif and can load RAW CT images from a CT scanner directly.
GridFlow is a plugin for PureData, Ruby, and jMax. It provides the ability to process images and video as N-dimensional arrays ("Grids"). It consists of a framework for grid transmissions, elementary grid processing objects, picture-oriented processing objects, and input/output objects. It also allows one to write plugins for PureData and jMax using the Ruby language.