Justin has been using this one for forever:
http://www.gimp.org/
I can't really help you with it, though. It has all the base Photoshop tools but the layout is completely different. Justin just now switched to Photoshop and was like "WHERE IS EVERYTHING" but he also knew what everything was so it's a good teaching tool. Once you get Photoshop I can help you make the transition.
--
~Zen