"...That's not GIMP" is really the key phrase here; other free art programs aren't mentioned as often and aren't as widespread as GIMP because, simply put, they are lacking in comparison to what GIMP has to offer.
I mean, you could try browser editors like Pixlr, or downloadable programs like Paint.NET, but the best free image manipulation program that's out there and proven to be good with graphics-making is GIMP.
Of course, this answer definitely wasn't the one you were looking for, but if you're serious about this then GIMP is the way to go. If not, I suppose you could try out the other programs I listed above and see which one you like.
(Oh, and if you wanted suggestions for programs that are good for drawing - GIMP can do that too, but I've seen people say good things about FireAlpaca and MyPaint. Anything else on [
this] list could be good as well.)