Hmmm, I'd have to go with Blood Metal from the Runelords series of novels, a bit of a guilty pleasure of mine. Basically, it is a rare metal that can be used to drain Attributes from one person, and given to another. Naturally, this is used by the nobles to make themselves and their knights much stronger, faster, and smarter than the average person, and there's lots of logistical fun to be had with it.