Simple image manipulation application
Pixelmator is a great image editor for OS X if you don't need all the bells and whistles of Photoshop. It's only $30USD, an order of magnitude cheaper than PS, and has a free trial.

I think this is the best choice for beginners or those with more lightweight needs. Because it's Mac only, it integrates with the core technologies of the OS better than even Photoshop does, so it's a very natural learning curve for an OS X user.

However, if you need something totally free, you can also check out the open source GIMP project, but this is starting to approach the complexity and power of something like Photoshop.