I recently downloaded GIMPPortable from http://www.portableapps.com and I have to say that it's quite amazing.

GIMP has always been a good free alternative to PhotoShop, but since it's main target was the Linux community, it didn't take off that well among PS users. It can pretty much do everything that PS can do and has some cool extra features.

What's nice about the portable one is that I have it on my USD HDD which I take with me when I go on a photoshoot and if I need to use someone else's PC, I'll still be able to edit my photos