When it comes to photo editing, the undisputed king is Photoshop. But if you're looking for a cheaper alternative that can give you the same – or at least similar – functionality then Active Pixels is an excellent choice.
This photo editor resembles Photoshop in many aspects, from the way menus and windows are laid out on its interface to the keyboard shortcuts used for many actions. It also includes similar tools, like the magic wand, the gradient, the polygonal lasso and others. What's more, it supports working with layers. Active Pixels also includes some extra embedded apps, such as a red-eye fixer, a picture browser and a screen capture tool - although it only grabs full-screen images.
That said, bear in mind that this free photo editor is not as powerful as Adobe's big shot. Despite the large number of filters and effects included, some of them lack proper configuration settings. I specially disliked the way sliders are designed: they're very uncomfortable to use.
In any case, Active Pixels is an excellent photo editor that can be very useful for those cases in which you don't have Photoshop at hand or you just prefer to use a lighter app to perform the same tasks.
A great photo editor with which you can edit and optimize images, all for free.