GIMP

The GNU Image Manipulation Program (GIMP) is an open-source image editor modeled on Adobe's Photoshop suite. While GIMP lacks a few of Photoshop's most advanced features, it acts as a Photoshop replacement program (and it's totally free, compared to CS4's $699 price tag). The GIMP website also offers an extensive community of users and programmers who answer user questions, troubleshoot errors, and publish tutorials.