Soundtrack
Avatars
Wallpapers
Screensaver
Resource Extractor (available http://quick.mixnmojo.com/software#psychonautsexplorer)
Renders (http://psychonauts.atspace.org/renders.html)
Comic series (http://www.psychonauts.com/developer.htm)
Stereograms (http://www.psychonauts.com/headgames.htm)
Demo (http://www.psychonauts.com/updates/PsychoDemo1_02.zip)
FanArt (http://psychonauts.deviantart.com/)
I know I could get most of this my self, but it would be nice to have it easily availible on gog.

I don't know if it's what you're looking for, but you can get an explorer to extract, view and convert resources from Psychonauts here.

That's exactly what I meant, I'll link that one instead, thanks.