The GIMP is a bitmap graphics editor, licensed under GPL.

Wikipedia's overview: http://www.wikipedia.org/wiki/GIMP for a general overview)
Official page: http://www.gimp.org/
A good manual called Grokking the GIMP by Carey Bunks is on-line.

Installation Questions

Q: Apparently ported to Mac OS X... but where and how?

A: The GIMP runs using the XWindowsSystem (not to be confused with OS X). X Windows is a graphical environment for unix; as OS X is unix based, X Windows (and The GIMP) will run on OS X.

One way to get all the necessary stuff installed is to use the FinkProject (http://fink.sourceforge.net/). You can use fink to install the XDarwin program and the GIMP. Or, Alternatively, install Apple's X11 program and then install the GIMP.

MacGimp (http://www.macgimp.org) specializes in CD-ROMS that install the GIMP and all its requisite parts. This may be an alternative for people who don't want to learn how to use Fink.

If you are looking for a GIMP with an aqua interface, rather than X11, keep an eye on CinePaint (an offshoot of FilmGimp, which is an offshoot of TheGimp) it uses the GtkOSX API