Gimp is a free open source image editing program available for just about every platform which includes the Mac, Linux, and Windows. It’s arguably the best free alternative to Photoshop. What makes Gimp so popular is the large user base who are very knowledgeable.
GimpShop is a version of Gimp which was created to act and feel like Photoshop. If you’re a seasoned Photoshop user then Gimpshop was made for you to feel right at home.
Seashore is also open source like Gimp but unlike Gimp it’s only available on the Mac. Based on the Mac Cocoa framework this image editor features most things you would expect from a great image editor like textures, gradients, and multiple layer support.
ChocoFlop image editor uses Apple’s CoreImage technology. One of the features of this editor is how lightweight it which makes it super fast even on older Mac hardware.
iPhoto is a free basic image editor from Apple available for Mac users. Not really for a power user but its good for basic stuff like adjusting brightness, scaling, cropping, and fixing red-eyes and managing your images on a Mac.
Preview on the Mac is more than just a image viewer, and it’s good for basic image editing. To see what it can do when viewing an image inside Preview go to Help > Preview Help.