GIMP is the GNU Image Manipulation Program. It runs on multiple platforms and has similar functionality to Adobe Photoshop. It is a fully capable image editing program, very useful for tasks like cleaning up images from your digital camera, converting between formats, etc. It can be scripted, but does not run Photoshop scripts.
GIMP is released under the GNU General Public License.
http://www.gimp.org/tutorials/ (gimp tutorials)
http://gimp-savvy.com/ (website based on the book "Grokking the GIMP")
http://www.tasi.ac.uk/advice/creating/gimp.html An excellent tutorial on Using Gimp software for teaching and research from TASI, the Technical Advisory Service for Images, another great JISC-funded advisory service
Are you using GIMP in a UK college or university? Add your use stories here.