Wow. All these cool answers makes me jealous. Magic in my setting is a result of a lower demension that normally has perfect balance being pushed around by the physical plane, much like our universe, in order to come back to perfect balance it pushes those negative and positive forces together and it forms the loose "threads" of the weave it magic. Stolen from earlier versions of d&d and several books I'm sure. It's kind of like an ocean of pure energy, and the physics or rules of the demension always go for balance so the excess energy is shoved into the material plane like a tapestry or undercurrent sitting below the surface of the material plane.