GIMP for Mac OS X

Since the 2.8.2 version GIMP runs on OSX natively. No X11 environment is required.

Native builds

Currently there are two main installers:

A GIMP 2.8 DMG installer by Sven Claussner, stock GIMP build without any add-ons. It works on OS X 10.6 Snow Leopard and later. Just open the downloaded DMG and drag and drop GIMP into your "Applications" folder.

A build by Simone Karin Lehmann, with some extra plug-ins and changes.

Macports

A (easy?) way to compile and install GIMP and other great Free software on your Mac is by using Macports. The installer allows you to choose from a large directory of packages. To install gimp using Macports, you simply do sudo port install gimp once you have Macports installed.

Disclaimer: we have not investigated if it's feasible to build GIMP 2.8 from Macports. The GIMP port file mentions 2.8.10, and appears to be maintained, however.

Homebrew

Homebrew is similar to Macports and provides packages (aka formulas) to install, either by compiling them from source or by using pre-made binaries. While there appears to be no formula for GIMP itself, there are reports that all dependencies are available. This may help to build GIMP from source.

Fink

Fink is a package repository that offer mostly precompiled binaries. It provides the apt-get command known to e.g. Debian and Ubuntu users, and installing GIMP is as easy as sudo apt-get install gimp once you have installed the Fink installer.
If there's no binary package, then fink install gimp will compile GIMP from source.

Disclaimer: we haven't been able to determine if it is possible to install or build GIMP 2.8 from Fink. GIMP 2.6.12 appears to be the most recent GIMP package that is offered there.