I don't use Gimp. I know people who do, so it seems to be pretty good.
50 Tutorials To Get You Started With Gimp | LinuxHaxor.net:
"Gimp has been famously known as the “poor man’s Photoshop”, and perhaps rightfully so. That’s a complement Gimp won’t mind taking. While it would be an unfair comparison to make between Gimp and Photoshop, Gimp can easily meet needs of most amateur image editors out there and then some. Since its release in 1995 Gimp has come a long way in to being the most powerful image editing tool freely available out there. With these tutorials we hope to vanish some of the doubts you might have had about Gimp’s ability as a powerful image editor."