Although I do agree with the suggestion of the Dresden Files RPG magic system as one to definitely consider, there is another option that I would put forth - The Fate of Inglemia. It's not as wide open as the system from DFRPG- being more of a Vancian system with definite drawbacks. It also builds on, rather than replaces wholesale, the magic system in the FATE 3.0 SRD. But that framework to me gives it a bit more flavour- and this is coming from someone that absolutely loves the system in DFRPG.
The system itself is built for High Powered magic at the higher levels, so competes with the DFRPG on that point. It is based on skills and aspects, with the aspect giving the ability to cast spells (and increase that ability), and the skills defining the area of knowledge that the caster knows, how well and how much he can cast spells from that arena, and his general casting abilities.
When creating a magician, the first thing that you look at is your aspects. Every magician must have the Magical Talent aspect, and at least one level of the Magical Initiation aspect. Each additional level of the Magical Initiation aspect increases the circle of spell the magician has access to for all Schools which the mage has skill in. Other magical aspects can be purchased also in relation to the character's background as usual for a FATE character. These aspects are important in spell casting.
The next thing to purchase is skills in magic. The general FATE 3.0 SRD skills of Evocation, Summoning, Mastery, and Dispelling are used to control your spells. Skill in the initiated school of magic must also be purchased. In addition, a mage that purchases an aspect to define his specialization can purchase skill related to that aspect, and stunt ranks. The skill is useful for out of combat uses of the specialization in unique ways, and the stunt ranks can be useful in creating unique uses of the specialized magical skill in combat, and protection against feedback if the mage pushes his powers.
As I said, the system has Vancian characteristics - at the beginning of the day, the caster must study spells from a spellbook; the muber of spells in a given circle which the mage can have prepared is equal to the number of skills ranks invested in the skill. Each spell cast successfully marks off a rank of any magical aspects, limiting the number of times per day a mage can cast, and the spell is also lost until prepared again.
If all of a character's magical talent aspects are checked off, and the mage still needs to cast a spell, difficulty is determined and the spell check is made to cast the spell. If it is successful with a MOS of 1, the spell is cast successfully as normal. If it is failed, or succeeds exactly, the spell is treated as an attack on the character with a severity equal to the difficulty of the spell, plus the amount the casting roll was failed by. The only defense against this attack is the character's magical talent, and any stunts that he has that are directly related to the spell that are being cast, i.e. a pyromancy specialist would get to use his pyromancy to defend if he was casting a fire spell.
There are problems and areas to be cleaned up; I had to read several times to get an understanding, and I'm still not sure if its the author's intent. There were also several clarifications over time in character creation and play, as several times the rules seem to contradict themselves. But it's very flexible in action, while having a more defined framework than the DFRPG magical system. The background itself is also quite well done, and it really shows that the intent was for a high powered game with definite strategic efforts like Birthright, Ars Magica, and The Primal Order.
