For the ignorant (and there exist many), GIMP (General Image Manipulation Programme) is a free and open source image processing tool. It has nearly revolutionized image processing mostly owing to the fact that it is free unlike close rivals Adobe Photoshop Extended (which is about $1000 more expensive).
Adobe Photoshop and Gimp, both are however different and unique in their own ways. For a comparison between the two, head to FindTheBest.com and a simple Google search away.
But this is where I state the most obvious difference GIMP, unlike Photoshop, works on UNIX and Linux (It was initially made for this OS's, ported into Windows and Mac much later). And for those operating systems, it is the best image processing software around without question.
The professional and rich still use Adobe Photoshop, mostly because of historical reasons, but the geeks and poorer amateur artists have been slowly gravitating towards this open-source tool.
But the bridge between the 2 softwares has been narrowing down. (See GIMP 2.6 released, one step closer to taking on Photoshop.) Though GIMP would like you to believe that they don't give a damn about the rivals, and whatever they do, it is to get closer their own product vision.
For any software to survive, it needs to create a community. It is this community which supports each other, lobbies for updates and most importantly troubleshoot - debugs and improves the software. This tool was created in ways to engage the user community - it had an active plug-in system, so developers could make separate programs to add to GIMP without having to alter the main distribution. But they went a step further and actively engaged the developer community, creating a mailing list which allowed everyone to interact on an open forum. It was a success. The tool grew fast, improving with every iteration. Soon the list broke up into gimp-user and gimp-developer, differentiating between the back-end code and the image manipulation itself. This is key. For a tool to work, people need to be able to use it effectively. Just creating a bug-free tool isn't enough. You need to educate the public. And this endeavour was accomplished by people putting out tutorials and people sharing their artwork. On October 7th, 1997, two users, Karin Kylander and Olof S., announced the Gimp Users Manual.
GIMP initially used the commercial Motif widget library for the core windowing capabilities. This of course went against the ethos of the open-source movement. It eventually led to the creation of an independent, open widget set based on an open core drawing library - The Gimp Toolkit (GTK).
GIMP is primarily developed by volunteers. And for a software running on willing unpaid labour to succeed in a crowded marketplace truly makes this software special. It is released under the GNU General Public License (version three or later) as free software, and works with numerous operating systems, including GNU/Linux, Mac OS X and Microsoft Windows.
Hopefully this software will continue optimizing itself, and show the robustness of community networks in creating tools as compared to rival more centralized 'big-corporation' softwares.
Wilber is the official GIMP mascot, created by Tuomas Kuosmanen (tigert).
No one is quite sure what animal the mascot is supposed to be.
A Brief (and ancient) history of GIMP
The GIMP vs. Adobe Photoshop, or ``Davy and Goliath''
The GIMP Plug-In Registry allows authors to update their plug-ins, and people to register their plans for future plug-ins
Grokking the GIMP, by Carey Bunks