If you have a Mac and want a powerful image editing app which doesn't cost a bomb or are looking for a cheaper alternative to Photoshop, Pixelmator is one of the best.
Pixelmator was recently updated to v2.1 adding a bunch of new key features. The new features include: retina ready, iCloud support, effects browser and alignment guides.
Probably the best thing about Pixelmator is that it is only £10.49 (for a limited time only). You can do nearly everything Photoshop can, without having to pay £667. That £10 seems nothing compared to that.