Pixelmator has always been popular for the not-quite-ready-for-the-expense-of-Photoshop crowd. The image manipulation/retouching app for Mac is powerful, works well, and is good competition for applications like Photoshop Elements.
Pixelmator 2.1 has just been released, and at a very attractive price through the Mac App Store at a very attractive US $14.99. The app is now Retina-ready, supports iCloud for syncing, and has added new effects including miniaturize, rain and snow and new filters. The app also supports Mountain Lion.
