Does anyone know which items trigger interesting NPC actions?
I don't mean the custom made items that require some ingredients.
I mean giving Ferro a jar of conserved trynnie results in obtaining the armory key. And how do you find out? Anyone had a conversation with Ferro where he commented on liking pickled Trynnie? (yuch)
The only other interesting gifts I found (searching this forum) were a diamond for the bank lady (quest, doesn't really count) and booze for a rapax constable. I can imagine that giving certain types of animal parts to a potion maker will earn you something in return.

Anyone tried giving NPC's items and got results other than their "gratitude"?

No. Its Crock who says Trynnies are good eating.
And you don't get anything for giving him the pickled trynnie.