I'm currently working on a low-magic campaign. It's taken over a month for me to work out all the problems that arose from tweaking the magic system in D&D.
After a failed attempt to achieve similar results with Spell Rolls, I basically did a port of the Unisystem Buffy the Vampire Slayer magic system over to D&D3e. This allows for spell failure, spells per day limited by fatigue and repeated casting, as well as wild magic.
Due to changes like the average mid-level spell caster only being able to get out 8 or maybe 9 spells per day without a serious mishap occuring, I had to tweak the spellcasting classes a bit.
Here are some of the changes that I had to make. Sorcerers have a d6 hit die. Wizards have an 8+ base for skills instead of a 2+. They also use the Bard's class skill list in addition to their own. Clerics have a base skill points of 4+ instead of 2+.
The various spellcasting classes also receive a random low-level power. The sorcerer's power list is a bit higher in power level, but they also are the only class with a restriction upon their powers.
With the system in place as is, I can use spells from the Player's Handbook, EverQuest, Diablo II sourcebooks and Call of Chthulhu d20 interchangeably. The powers also help to displace a bit of the spellcaster's loss of firepower. For what they didn't address, I boosted the base DC of a Save vs. Spells from 10 to 15.
In order to offer a wider spectrum of fighting styles and address the influx of fighters and rogues into the gaming group, I have also adopted many of the rules outlined in the Swashbuckling issue of Dragon. There is also the option for characters to go unarmored and use a Defense Bonus by Class similar to that found in Star Warsd20.
In order to make up for the new randomness in healing magic (it's possible that a Cure Light Wounds gone wrong could turn a character into a dying chicken), I have implemented the Reserve Hit Points rules found in the Omega World Polyhedron by Johnathan Tweet.
The total document is about 40 pages typed, but many of the rules are pulled from a variety of sources and the Unisystem magic looks like it will be a lot of fun. We'll see what the players think of it though...