Check out Pixlr
Pixlr is an amazing photo editing software. It's essentially set-up like Photoshop just without the more powerful features. It's completely free and if you know how Photoshop works, very straight forward)
My only issues with it, is that I use a large amount of keyboard shortcuts- and they don't work on this web-based software. I also had a difficult time adjusting to the subtle differences in layout.
I love Photoshop, and our computers in the lab all have them- however when I'm running around town, or using another computer- I cant afford the program. So Pixlr is a GREAT alternative!