Where to Get the GIMP

The GIMP homepage is at http://www.gimp.org/ where you should follow the "Download" link. This will get you the latest version (v1.2.3 at this writing) in tar.bz2 format, and you'll have to compile it yourself. I generally use precompiled binaries - these are available for just about every Linux distribution, and usually installed by default.

The Windows version is available at http://www.gimp.org/win32/ and http://www.wingimp.org/. The latter is a link from gimp.org, while the former is a more complete and nicer looking project page. I believe they're both maintained by the same person. They're currently offering version 1.2.4 prerelease.