I haven't used any of these heavily, but they come up often. Over the past few years I've heard a lot of good things about Krita[1] as an alternative to Gimp. I've found both Pixelmator[2] and Acorn[3] to have enough of the features to replace Photoshop for basic needs.

[1] https://krita.org/en/download/krita-desktop/ [2] http://www.pixelmator.com/mac/ [3] http://flyingmeat.com/acorn/