Witchery

Witchery is a unified magic system, designed to be adaptable to most traditional tabletop roleplaying systems. Each spell in this system is built in a conversation between player and Referee by combining an element (like fire), an art (like conjuring), and a conduit (like brew). However, these components don't dictate the spell exactly; a fire-conjuring brew might be a blackened lump that explodes on any hard impact, or a glass bottle that sheds light for a few hours when shaken up. Magic Dangers will also complicate the casting.  

As a system, it includes a significant number of hooks onto which you can attach mechanical effects from your rules engine, or which you can leave open for narration instead, as desired.

This magic system is focused, like most RPG magic systems, on a list of magical effects that are in practice organized into spells, and notes on their creation. It has been structured to feel more akin to witchcraft or folk magic than to the Vancian wizardry most traditional in RPGs; not a great rarity, but worth noting.  Well-read gamers who have encountered Ars Magica or the Verb/Noun casting system of GURPS Magic will find this system very familiar; it is one of their descendants (and proudly so), though far lighter in structure than either.