Inkscape and GIMP are two different types of graphic applications.

Inkscape is for vector graphics. It is similar in concept to Adobe Illustrator. It has some great functionality, a few annoying bugs, and is definitely worth trying. Version 0.47 is just about to come out. Search for the tutorial videos by heathenx.

GIMP is for bitmap graphics, similar to Adobe Photoshop. I haven't used it, but I think I've heard good things about it.