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".
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