JMLEONFOUNLIN  PhotoLine 15 may not have as beautiful an interface as Pixelmator, but it is very powerful. As far as I know it is the only editing photo app which allows you to use most editing tools in Lab (without the need to convert the image to Lab, though you can so if you will) or HSV-HIS mode directly within the tools. Raw Developer also permits editing curves not only in RGB but also in Lab (but it is a raw converter). It also supports CMYK.

Contrary to what has been said there is a history palette, but PL doesn't have quite the same terminology as PS (View -> Dialog -> Undo List). Sharpening can be done non destructively (on a layer). The app also makes it possible to use screen and printer ICC profile (I use Spyder3 Pro and PrintFX Pro). It can also create HDR images and can import raw images.

Recently I've done all my editing in PL 15 up to framing and watermarking and exporting for web (leaving raw editing to Capture One 4 though). Really, this app deserves much more than the review I've read here but you have to go beyond the not so tasty face. Remember that it is also relatively cheap. For all the features, it is a real bargain in fact.  
(Version 15.01)