For generating normal maps there are several tools available… “available”…:
Bitmap2material: $149
Nvidia’s Photoshop Plugin: You need Photoshop (~$800)
But finally I searched for a normalmap generator for GIMP, and this is what I found:
http://code.google.com/p/gimp-normalmap/#Overview
It’s truly awesome… (I’m on Archlinux here, quickly installed the tool with $ yaourt -S gimp-plugin-normalmap, started gimp, and it was there…)
Using the plugin is extremely easy:
You can then specify options:
You can even let it perview the bumpmapped image with lighting as a quad, sphere, torus, whatever:
With this tool I converted for example this carpet image:
Into this normal map for it:
These guys did a great job!