It sounds like all the obvious things about Gimp vs. PS have been brought up.
Another alternative I have used is called Paint.Net.
http://www.getpaint.net/index.html
I am no PS expert by any means, although I do have a copy and have used it. I find Paint.Net easier to use than PS. It just seems more intuitive TO ME.
It is for Win only (sorry Mac!). I would love to hear any feedback regarding "PS vs. Paint.Net" from members have have played with it.
Cheers,
Paul