I have collected skulls, bones and scrags to use as components for the Animate Dead spell (Skeletons), but unfortunately it doesn't work...
A brilliant idea. A pity it doesn't work.
There are also special jewelry and journals that can be found, but no idea if relevant NPCs / quests can be found for this later in the game.
I read somewhere else that reading a book makes you memorise its contents. That lead me to the conclusion that I can safely sell all the books I have read. Except books with a special border colour, of course.
In some cases, the weight of an item seems worth paying attention to. I keep items with special names and no weight in my invisible bag, because I think the developers are telling me to hang to those things by giving them zero weight.