I just ran across this and was blown away by what I saw on the tutorial page...http://www.philipk.net/ndo.html
I don't have time right now to tinker with it. I'm wondering if this works with GIMP. Probably not.
Anyway, enjoy guys.
I heard PixPlant is also good. Generates both Specular Maps and Normal maps, displacement maps from plain photoshttp://www.pixplant.com/