I recently found myself tinkering with graphics for various projects, and GIMP served me well as it has consistently for nearly 10 years, across numerous Windows and Linux versions.
What surprises me is how many people remain unaware GIMP exists – and what a great tool it is. Of course, full-time graphics and photo professionals typically sidestep GIMP for Illustrator and Photoshop, just as I use Visual Studio instead of SharpDevelop for my .NET development; but for me, others whose work calls for occasional graphics hacking, and many hobbyists' needs, GIMP is a wonderful, free tool.
If you've never heard of it or used it, embrace GIMP!
* Screenshot courtesy of the GIMP team under a Creative Commons Attribution-ShareAlike 3.0 Unported License.