True to its name, this popular and affordable app's main functionality is converting graphic files--importing from some 200 formats and exporting to about 80 at last count, with a tremendously time-saving batch processor thrown in for good measure. But GraphicConverter also proves invaluable as a high-speed image editor that lets you quickly browse, resize, sharpen, adjust color, and optimize for the Web, all at a price that makes commercial packages blush.
GraphicConverter's interface hasn't quite kept up with its ever-growing feature set (and many of its most useful features can be accessed outside of the app anyway, through contextual menus), but what this app lacks in simplicity it more than makes up for with copious features. The app also includes extensive documentation, and the program's uber-diligent programmer (whom even Apple has called "one of the most conscientious developers in the shareware universe") provides frequent fixes and updates. Recent changes have included the ability to view images in a "digital light room" browser mode, a completely rewritten backend for support of 64-bit, added support for large images with more than 16,000 X 16,000 pixels, and many more fixes and minor feature enhancements.
If you're looking for a low-cost way to make quick edits to images or batch convert to other formats, this app is a no-brainer for Mac.