The Gimp is the best. Free & legal download (it's Open Source) at http://www.gimp.org
Added - I've got that Photoshop somewhere. The Open University provided it on disk with their photography course. It doesn't work with my operating system, Linux Ubuntu. However the lessons were easily followed using The Gimp instead. The learning curve must be the same in that case then, no?
Added - The Gimp has no limitations even though it's free and has all the digital editing functionality of all the paid for digital editors. Open Source is a brave new world people don't understand, try some Open Source software and see for yourself. I recommend Ubuntu for an operating system, The Gimp for digital editing, Open Office for office software, Firefox for a browser, etc...
Of course, you can just try The Gimp for yourself now, in a couple of minutes... It's a free and quick download, full version with full use. As someone said the full version of Photoshop is over £600, for the same thing as The Gimp.