Well GIMP is the best and most advanced raster image editor for free - nothing else comes close, but it's not the only free option in town. GIMP wasn't really designed as a drawing tool, it's main purpose is for editing photographic images, however you can still use it to draw, and you can also customise GIMP for digital art by getting GPS (GIMP Paint Studio) which is an addon for drawing and painting. It's free here: http://code.google.com/p/gps-gimp-paint-studio/ - don't forget to check out the guys videos and gallery.
Other software that I can recommend specifically for drawing/illustrating:
MyPaint - Open Source raster drawing application (similuates natural media like pencil and watercolour paints etc) http://mypaint.intilinux.com/ - simple but absolutely brilliant especially if you use a graphics tablet.
Inkscape - Open Source vector image editor similar to Adobe Illustrator. http://www.inkscape.org - seriously good software. Inkscape is to Illustrator, as GIMP is to Photoshop.
Paint.NET - simplified image editor, not a patch on GIMP's capabilities, though it might do you just fine. http://www.getpaint.net - although this is not really a drawing application, it's an image editor. Great for those who find GIMP too complex.