Krita

Krita is an application mainly focused on digital drawing, although it can also be used as an image editor, or even as a vector graphics tool. In the beginning it was developed for Linux as part of KDE and included in Calligra Suite. This version offers all the features of the original application, but now for the Mac environment.

The interface style is influenced by Adobe products, as well as by Corel digital drawing tools, so anyone who has used Photoshop before can quickly get the hang of it.

It includes a lot of different brush types and effects, the ability to work with layers, support for the most widely used image formats, palettes, cropping, and vector manipulation, as well as different retouching tools.

The best thing about it, though, is that it's an open source application under GNU license. This makes it a great alternative to Gimp, the most popular open source image editor by far.