Every once in a while, you stumble upon some real gems in your blog subscriptions. Recently, Raph Koster, author of "A Theory of Fun," finished blogging a series of articles that describe the process they used to create the resources system used in Ultima Online. Some interesting stuff in there from a virtual world design standpoint. Here are the three articles:
Part One is about the underlying data structures.
Part Two is about the application of those data structures to the virtual world.
Part Three is about the future directions they didn't get to pursue with UO, and an ample source of ideas for your own creations.
Those articles reminded me of a permeating magic discussion that was going on the RPGDX bulletin board a while back that generated some interesting ideas for a magic system for RPG's.