Mac: Pixelmator has always been a great alternative to Photoshop thanks to its enormous feature set and low price. Not only does the latest update bring a bunch of new features, but it also reduces the cost of $60 to just $16 for a limited time.
The new version brings support for retina displays and OS X Mountain Lion. On top of that, new image effects and alignment guides make this a really great time to check out Pixelmator (or update if you've already got it).
Pixelmator Adds Support for OS X Mountain Lion [Pixelmator]