GIMP (GNU Image Manipulation Program) is a free software raster graphics editor. It is primarily employed as an image retouching and editing tool and is freely available in versions tailored for most popular operating systems including Microsoft Windows, Apple Mac OS X, and Linux.
Any self-respecting Linux user or on that note, any self-respecting graphics designer should know about this program. It is *the* open source graphics and image editing suite and has been developing into a worthy contender for years. In fact, it could be said that it has been developing alongside commercial rivals such as Photoshop.
Some of the tools you can use in GIMP are the paintbrush, pencil, airbrush, clone, gradient, paint bucket and blending tools. If Photoshop has it, you can be GIMP has it or at least has some kind of replacement in the development tree. Like Adobe, GIMP for Windows also includes support for layers and plugins. In fact, you can install up to 100 different plugins in GIMP for Windows.