GIMP. There's even an add-on for GIMP to make it look like Photoshop, it is called GIMPshop. Here's a review that I made http://alternativefront.com/alternative-photo-editing-software/gimpshop-gimp-with-a-familiar-look
MS Paint xD If you are very talented designer this kind of tools won't be a problem xD I have seen that one man was drawing Mona Lisa in Paint -.^ maybe it's still available on youtube So..Photoshop is exactly the same as Paint but..yhmm faster xD
GIMP hands down! But its if you actually want to do serious design work than you must invest in Photoshop, but for webmasters, etc that only need to edit a few things every now and then it is perfect