From GIMP's website:
"This is only a very quickly thrown together list of GIMP features. This is only the tip of the iceberg.
* Full suite of painting tools including Brush, Pencil, Airbrush, Clone,etc.
* Tile based memory managent so image size is limited only by available disk space.
* Sub-pixel sampling for all paint tools for high quality anti-aliasing
* Full alpha channel support
* Layers and channels
* A Procedural Database for calling internal GIMP functions from external programs as in Script-fu
* Advanced scripting capabilities
* Multiple Undo/Redo (limited only by diskspace)
* Virtually unlimited number of images open at one time
* Extremely powerful gradient editor and blend tool.
* Load and save animations in a convenient frame-as-layer format.
* Transformation tools including rotate, scale, shear and flip.
* File formats supported include gif, jpg, png, xpm, tiff, tga, mpeg, ps, pdf, pcx, bmp, and many others.
* Load, display, convert, save to many file formats.
* Selection tools including rectangle, ellipse, free, fuzzy, bezier and intelligent.
* Plug-ins which allow for the easy addition of new file formats and new effect filters.
* Over 100 plugins already available.
* Supports custom brushes and patterns
* Much, much more! "
http://www.gimp.org/the_gimp.html
At first the interface is confusing, but after you figure out the right click menu and the selection quirks you should be set.
Carey Bunks "Grokking The Gimp" is very good: http://www.gimp-savvy.com/
but as I write this its website is down for repairs due to a hard drive crash.
other good places to learn more about GIMP are:
http://gug.sunsite.dk/
http://wiki.gimp.org/gimp/
http://www.rru.com/~meo/gimp/
Now GIMP to your heart's content.