All of us open source guys have been trying to get GIMP the attention it deserves for years. While the learning curve on GIMP is a bit more steep than Photoshop, once you get the hang of it the program has just as many capabilities as Photoshop without the bulk and resource hogging.
The best part, it is distributed, as you said, for free in almost all major, and most minor, linux distributions.
I can give you some good sites with info, tips, and tricks for GIMP if you want.