This is something that's been a long time coming, and it's finally here. GIMP is an excellent Photoshop-esque open source photo editor. While it's been available on Mac for a long time, it has required the X11 Window environment, which itself needs a separate (and somewhat messy) installation. For a long time, the programmers working on GIMP have been promising to eventually take it native, but it hasn't happened until just recently. As of version 2.8.2, you can now simply go grab the GIMP .dmg file from the main website, and then install and run on your Mac as needed.
That's good news for those of us who appreciate a powerful photo editor like this but don't necessarily have the cash for big-name apps like Photoshop or Pixelmator. Congrats to everyone who's worked on GIMP in the past. If you like the app and want to support great open sourced work, they could probably use a donation or two.