According to Wikipedia:
GIMP (The GNU Image Manipulation Program) is a free software raster graphics editor. Primarily, GIMP is used as a tool for photo manipulations, such as resizing, editing, and cropping photos, combining multiple images, and converting between different image formats. GIMP can also be used to create basic animated images in the GIF format.
The product vision for GIMP is to become a high-end graphics application for the editing and creation of original images, icons, graphical elements of web pages and art for user interface elements. GIMP can also be used as a platform for the development of cutting-edge scientific image-processing algorithms.
The reason I use the Gimp is because I do the majority of my development on a Linux systems, and it saves me (and therefore my clients) a lot of money.
Below are a list of my most useful resources, when I'm working with the Gimp for web design.