I have Neb the dwarf's head. I took it back to Unger Hilldark expecting some reward or at least acknowledgement that I killed this dwarf that robbed him. The only response he has for me is to stay away from him. What did I do? What do I do with this stupid head.

Depending on which dialog path you took with Unger in your initial encounter, it's not necessarilly clear what you should do with the head, but try reading about it in your Journal and see if you can figure it out. Answer below:

SPOILER........................................... ...............................
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Turn it in to Chief Inspector Brega (sp?) at the back of the Government building for a reward and a rep boost.