GIMP is free!
As far as open-source alternatives to mainstream software are concerned, GIMP is the daddy. Mac-compatible builds of GIMP have been around as long as OS X itself. The difference now, however, is that GIMP now runs natively on Mavericks and Mountain Lion, and doesn't rely on X11.
Both its user interface and features have also improved significantly in recent years. While its obvious you’re not running Photoshop, many of the familiar palettes, tools, and menus are there. There’s the lasso and magic wand for selection, pen for drawing Bézier paths, paintbrush and a clone stamp. Adding layers is straightforward; just display the layers palette by going to the Windows menu and then Dockable Dialogs.
There’s something of a learning curve here. While many of the functions are found in Photoshop, and most are used by people who use Photoshop, they’re not necessarily where you might expect to find them. The Levels dialog, for example, is tucked away in a menu called ‘Color’ rather than Photoshop’s ‘Image’ menu. And Unsharp Mask, while in the Filters menu, is tucked away in a sub-menu called ‘Enhance’.
There are areas, however, where GIMP’s user interface is an improvement on Photoshop. Open multiple images simultaneously, for example, and while they open in one tabbed window, the tabs are represented by thumbnails of the image.
It would be wrong, however, to suggest that GIMP is a complete replacement for Photoshop. CMYK support, for example, is lacking compared to Adobe's application, there are none of Photoshop Extended’s 3D features, and there are no support for adjustment layers. And the less said about GIMP's Text tool, the better.
On the plus side, many of its included filters are really rather good. We were particularly impressed by Weave, which creates a reed basket-style weave pattern from an image, complete with offset sections and shadows.
Much of what’s missing can actually be added by way of third party plug-ins (or scripted yourself, if you’re comfortable enough using Python). There’s an active community of users and developers producing new filters and features. If there’s a tool you need or task you want to perform that isn’t catered for, there’s a good chance a quick web search will turn up either an extension or a workaround to do what it is you want to do.
GIMP was once used by Linux fans to demonstrate why they weren’t missing out because of their choice of platform (and it also used to be said that it was great for Mac users who needed to prep images before uploading them to a website). But it’s a great deal more than that now. The combination of some pretty powerful features, a decent UI, and great support makes it a very good choice indeed.