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/#OverviewIt'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:

[size=20pt]These guys did a great job![/size]