Not a Photoshop killer yet, but Pixel is an interesting low cost (US$32) alternative for your image editing needs. The software runs on OSX (no Universal Binary yet though) besides many other operating systems. It's a fast and small application that supports most scanners, printers, tablets and image formats. It can do layers and most color modes including 32bit/channel images that can be edited in full HDR quality. It has layer effects, some special animated brushes, retouching tools and many filters.