If you're a Mac user willing to shell out a few bucks, the $29 Pixelmator is a solid image editor for the price. But if you're editing images only every week or two, and you're looking for a free program, Seashore is the application for you.
Web software: Of course, if you're really unlucky you may not have access to your regular computer when you need to edit an image. If you're tasked with editing images while you're on the road or otherwise away from the office, try Pixlr, an entirely Web-based image editor available for free on the Pixlr website. For a Web editor, Pixlr is surprisingly powerful and feature-rich, with everything from layers and filters to standard tools such as the lasso and magic wand.
Unsurprisingly, Pixlr is also fairly poky. If you need to edit a large batch of photos, Pixlr can easily slow to a crawl, and even basic filters take significantly longer to run in Pixlr than they do in a traditional application. Nevertheless, if you're backed into a corner, Pixlr can be a lifesaver.
While professional designers and illustrators still ought to invest in the full version of Photoshop, most users should be able to benefit from any of the free image-editing tools and services described here. Whether you're making quick fixes or working on a few modest projects, these programs will meet your needs.