Ironically, it's free: a review of GIMP 2.6.4
As the open-source image editor of choice, the feature list of GIMP 2.6 is very long, and despite its status as a free application, it's as feature-packed as any commercial application. GIMP is loaded with the up-to-date tools many demanding professionals need: Bézier path editing with brush stroking, tablet support, Heal Tool, alpha channels, multiple-undo History, area-averaged eyedropper, PSD file compatibility, and a wackload of other high-end tools that are impressive in their attention to detail.
Test system: MacBook Pro
Bring Out the GIMP
Before I started throwing serious workhorse tasks at GIMP, I needed to set up a few important tools: Intuos3 tablet support, my brushes, and color calibration. On OS X, where X11 isn't the default window engine, this proved to be a bit tricky. After a bit of digging around, I found out that the X11 that ships with Leopard doesn't support pressure-sensitive tablet input, so I had to download the latest release candidate build of XQuartz (note that you need at least 2.3.2 for Wacom support). After enabling the Intuos mouse in the Input Devices panel in GIMP preferences, I was up and running: