Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
GMP, the GNU Image Manipulation Program, is a long-standing and hugely respected open source graphics program, and many readers probably already use it. Originally created at U.C. Berkeley its interface and feature set runs neck-and-neck with expensive proprietary alternatives such as Photoshop, and it has a thriving community of developers and plug-in creators. The GIMP site has many useful resources for the application, and there are also a lot of other places to visit for turning yourself into a power user of this excellent cross-platform application that always leaves new users bewildered that it is free. Here are six good choices.
Many GIMP users who use or have previously used Photoshop swear by GIMPshop, which is essentially a hack of GIMP that gives it an interface equivalent to Photoshop's, right down to individual menu choices and terminology used. In fact, it's so close in interface to Photoshop that, using it, you can follow the thousands of Photoshop tutorials available online in GIMP. Mac, Windows and Linux users can install GIMPshop.