I love this software. You can get it free online from their website (just search "gimp"). I originally started using it because I got a new computer and couldn't find my photoshop CD anywhere - but even now that I have a working copy of photoshop, I still keep it on my external harddrive and use it at school or the library or any computer I might use other than my own - because the software is small enough to be portable!
I have yet to find anything it can't do that photoshop can, I just find it a little tricky to navigate - though that could just be because I'm used to working with Adobe programs. If you need or want photoshop for any reason but can't afford to shell out a couple hundred dollars, this is the perfect substitute.
One thing I don't like about it is that it doesn't just open one window with all the toolbars in it like photoshop does - everything opens as a seperate window. So if you are working on a few different images at a time, they will all show up seperately in your taskbar and your tool bars will also be seperate windows.