In an effort to encourage more public participation in processing space images, here are a few helpful links for tools used by many image processors.
GIMP2 - Reverse Engineered Photoshop style program used in creating composite images.
MER Pancam Data Tracking Interface - Used to find out what operations were being done during each image sequence taken by the Mars Exploration Rovers.
Midnight Mars Browser - Easy way to download latest MER Raw Images, and also creates a virtual simulated environment based on those images.
MER Filenames - Using the filenames of the MER Rovers, it deciphers the information of the images in regards to Sol, Earth dates and time, filter numbers, equipment used, etc.
Gordan Ugarkovic's vmc2rgb software - Used to work with Mars Express raw images.
Microsoft Image Composite Editor - Easy to use program to seamlessly composite multiple images together to create smooth images from many.
Adobe CreativeSuite 2 - [PC/MAC] Adobe has ceased support of its CS2 software, but has released it free to the public, providing the serials to each of the programs. Useful software for image processing included in CS2 would be Photoshop for processing and panoramas and other composites, and LightRoom for other image editing.