Pixelmator

Pixelmator is an alternative to Adobe's Photoshop, built with just Mac OS X as the target system. It's slogan is "Image editing for the rest of us".

Major features
Pixelmator boasts many features including
 * "Worlds first GPU powered graphics editor"
 * Photoshop brush and file support
 * Quick look plugin (enables you to view more photos in it)
 * Based upon open-source components (ImageMagick)
 * iPhoto compatibility
 * Automator support
 * iSight easy import