GIMP.
It's very different, though, so if you're moving from Photoshop then you will probably have a tough time at first, but once you get used to it you'll find that you can do pretty much all the same things. The layout also gets some people (it doesn't use a single window to contain everything, although there will be an option for that in the next version), but again it isn't something that you can't get used to.