Some high-level neopagan did an analysis of the world’s magical traditions and came up with 26 common patterns. In a brilliant move of commercialization, he adapted them into a guide for designing magical systems for roleplaying games: a summary is available online. They’re probably invaluable to everyone from video game developers to Buffy scholars.

I’m going to use them to design an elemental magic system using urban elements (I have no idea what I’m going to do with this, which is probably why I haven’t done it yet). Although see also this mapping of all types of spells onto five elements.