This site has the above mentioned 5 alternatives to Photoshop complete with a quick review of each.
The Five are:
1. GIMP: A Great Photoshop Alternative and It’s Open Source
2. Pixelmator: Young Application Though Growing Strong
3. Corel Paint Shop Pro X2: Robust Application with Long Term Development History
4. PhotoLine: Lots of Features but Lacks That Mac Like Feel
5. Pixel Image Editor – Lone Developer Application
They also list some additional alternatives. Each has a link to the actual application or a demo version of it.
See it all here.
