Sure as the changing of the seasons, one such thread popped up a couple days ago, prompting myself and others to chime in with the usual round of responses. Specifically, the poster, disturbingly named irate fetus, was looking for "preferably something along the lines of DnD levels/spells per day." Fair enough.
Many responded to the call, including me, because I'm a sucker for that (and because it was pretty easy to plug Guy's Spirit of Greyhawk stuff on this very blog, which goes after AD&D-style "levels/spells per day" as a point of design). Building on something TheUnshaven suggested, and with helpful advice from TheMouse, I came up with something that also incorporated some of my own long-neglected, half-formed ideas. I think it's worth exploring here on the blog.
Suppose there is a magic-oriented skill called, I dunno, Magic. What you can do with this skill is determined by a magic-related aspect you have, like "Druid" or "Fire Mage" or something.
Give yourself a second skill pyramid that peaks at your Magic skill rating. So if you have Good (+3) Magic, your pyramid's apex is Good (+3). This is your spell pyramid. It has one Good (+3) slot, two Fair (+2) slots, and three Average (+1) slots. So far, they're all blank.
Every time you cast a spell, you claim one of the spell pyramid's slots, and use its rating as a bonus to your roll, just as if it were a skill. When you're out of blank slots, you're out of spells to cast for the day.
You know a number of spells equal to your Magic skill rating. Again, if it's Good (+3), you know three spells. Write 'em down. They only have to be names, and you only have to have a general idea about what they do -- don't sweat the details. So if your magic aspect is "Fire Mage," for example, your spells might be Fireball, Wall of Fire, and Flaming Bolt.
When you cast a spell, pick one of your spells and say what it's doing. It can Attack, Defend, Maneuver, Assess, or Block -- the standard FATE stuff. Whatever it is, it has to make sense within the confines of your general idea of what the spell does. You'll have a hard time Assessing with Fireball, for example, but an easy time Blocking with Wall of Fire. However, if you want to Attack with Wall of Fire, go ahead. It makes sense, after all, seeing as how it's on fire. You just won't be able to Block with it as well.
(Could you work in two effects, like Attacking and Blocking? Probably, if you spend a Fate Point. That seems reasonable. Still has to make sense, though.)
If you manage to get more than one magic aspect, like "Pyromancy" and "Divination," you get to write down another batch of spells befitting the new aspect, which means more types of things to do with your spell pyramid. How do you get magic aspects? Maybe with a stunt. That also seems reasonable.
You might prefer to have multiple magic-related skills instead, like a Pyromancy skill and a Divination skill, with attached aspects. This is perfectly natural and nothing to be ashamed of. In that case, the spell pyramid's apex is equal to the highest magic skill you have, but the rating of the spell pyramid slot for a given spell can't exceed its related skill rating. So if you have Good (+3) Pyromancy and Average (+1) Divination, you can use any slot for Pyromancy, but only the Average (+1) slots for Divination.
There is still the question of how these spell slots work, in terms of their ratings. Is that the only bonus you add to your roll, or do you add it and your Magic (or Pyromancy or etc.) skill? The latter seems a little unbalanced, although if you have a different skill for each sphere of magic, it also seems kinda necessary. Otherwise, those skills are just taking up real estate in your skill pyramid, and that's no fun. With a singular Magic skill, though, that skill could be used as a sort of magical Academics -- knowing about magic, as opposed to applying it. I suppose the same could be done with more specialized skills, but you probably wouldn't get as much use out of them.
The last piece of the puzzle is how to clear those spell slots so they can be reused. There needs to be a way. Obviously, sleeping for the night should do it, especially if we're kinda trying to emulate D&D, but I also want you to be able to clear them on the fly, in the heat of the moment. My initial ideas for that bordered on punitive -- that's our me! -- but TheMouse quickly set me straight by mixing together two of them.
The more mathematically sound way seems to be that you can clear up a spell slot with a rank equal to or less than the number of shifts your sacrificed Consequences are worth, +2 for each fate point expenditure. So if you do a Minor (2) and a single fate point, you can clear out a spell slot worth up to +4.That sounds pretty good.
So that's the gist of it. If I have a more concrete application for it at some point, I'll do more with it, but for now it's something for you to fold, spindle, or mutilate, as you will. Comments welcome, of course.