Imagemagick provides a rich feature set including:

• Format conversion: convert an image from one format to another (e.g. PNG to JPEG).
• Transform: resize, rotate, crop, flip or trim an image.
• Transparency: render portions of an image invisible.
• Draw: add shapes or text to an image.
• Decorate: add a border or frame to an image.
• Special effects: blur, sharpen, threshold, or tint an image.
• Animation: create a GIF animation sequence from a group of images.
• Text & comments: insert descriptive or artistic text in an image.
• Image identification: describe the format and properties of an image.
• Composite: overlap one image over another.
• Montage: juxtapose image thumbnails on an image canvas.
• Motion picture support: read and write the common image formats used in digital film work.
• Image calculator: apply a mathematical expression to an image or image channels.
• Discrete Fourier transform: implements the forward and inverse DFT.
• High dynamic-range images: accurately represent the wide range of intensity levels found in real scenes ranging from the brightest direct sunlight to the deepest darkest shadows.
• Encipher or decipher an image: convert ordinary images into unintelligible gibberish and back again.
• Virtual pixel support: convenient access to pixels outside the image region.
• Large image support: read, process, or write mega-, giga-, or tera-pixel image sizes.
• Threads of execution support: ImageMagick is thread safe and most internal algorithms are OpenMP-enabled to take advantage of speed-ups offered by the dual and quad-core processor technologies.
• Heterogeneous distributed processing: certain algorithms are OpenCL-enabled to take advantage of speed-ups offered by executing in concert across heterogeneous platforms consisting of CPUs, GPUs, and other processors.

Home Page: http://www.imagemagick.org/
Language: C with API's provided for C++ and Perl
Platform: Windows, Linux, OSX
License: Custom open source license that requires attribution. Free for commercial use. GPL compatible.