What are your favorite magic systems that you've read about, played with, seen, or even invented? I'm not talking about the generic magic systems, where 'magical' things happen because you said an incantation, or waved a wand, or something like that, I'm talking about the really well defined ones that have their own physics and whatnot that create a solid continuity. 

Also, what are your favorite uses of even the generic magic systems? Things like necromancy, but applied to things other than people and animals, bringing plants, stars, dreams, back to life; things like systems that work generically, but involve currency, in payment of your soul, or similar things...

Thoughts?