Gimp for Mac is a photo editing tool designed to be a free rival to Photoshop. This program has been popular with people running Linux for some time and is now available for Mac.
Free and powerful: Gimp provides almost as much professional-level photo editing power in a package that is completely free as premium products, which makes it an option that almost everyone should consider.
Similarity to Photoshop: The reality is that Photoshop has been on top of the photo editing world for some time. So if you're an experienced editor you've probably got a fair bit of familiarity with that program. Gimp has a number of familiar tools and interfaces that will be almost immediately clear to anyone who has used Photoshop before, which makes for a simple transition.
Interface flexibility: Gimp allows you to adjust the interface to your liking. If you're used to setting up your digital retouching workspace a certain way, you should be able to mirror that interface fairly closely within this program.
Automated selection tools: When you're editing large amounts of images, anything that speeds up your workflow is very welcome, such as automatic selection tools. However, these tools did not work exceptionally well in Gimp, and you usually have to resort to manually selecting your areas instead.
Although it isn't flawless, Gimp is a very powerful photo editing program. If you're looking for a cost-effective solution that will allow you to edit your photos with professional level results, then you should certainly look into Gimp.