INSANE BUMP is a plug-in for Gimp that can be used to create normal, height and specular maps from a basic color texture. In my opinion it is the best (free) alternative to CRAZY BUMP.
First you have to download the plug-in from this website:
Scroll down all the way to the bottom of the page and click on the link that says:
„InsaneBump-Windows-1.0.6.zip Plugin Executable. Contains InsaneBump.exe“
Safe the file so that you can find it later.
After you have done that, open up GIMP an go to EDIT –> PREFERENCES and click on the little plus button next to FOLDERS. Search for PLUG-INS and select it. You can now see the location where your plug-ins are placed on your hard drive.
Next you have to unpack the zip-file that you have dowloaded and place it in the folder that contains your plug-ins.
After restarting Gimp you should be ready to open up your first color texture. To start the plug-in, go to FILTERS –> MAP –> INSANE BUMP. Another Window will show up. You can tweak the settings and click on EXECUTE. The files are safed in the same folder as your first texture. Be sure to be patient, for some bigger files it may take a while to create all the textures.