Pixelmator has been a very popular app on Mac's purely because it's a Photoshop alternative at a dirt cheap price. It does nearly everything Photoshop can do and has a beautiful and flexible UI.
The app is constantly being updated, adding new and exciting features each time. It's a gorgeous and powerful app which can easily be an alternative to Adobe's Photoshop.
The app is half price at the moment at only £10. Considering the latest version of Photoshop is retailing at £626, you are saving a huge amount of money.