My Solution
Well, this is the solution that I came up with and used for a low-magic world a while back. We played in this campaign for a little over four months before all of the PCs were slaughtered by bears. (Yes, just normal bears.)
1) Ken Hood's Grim'n'Gritty Rules.
2) Paladin becomes a PrC. He gets more smites/day, no spells. They must have the "Inherent Magical Talent" feat, weapon specialization, and match the alignment of their deity. Any alignment, within the restriction that it must match that of the paladin's deity, was allowed. Evil paladins were referred to as anti-paladins, and replaced the blackguard. Detect Evil became Detect Infidel, smite evil become smite infidel, etc. and these powers worked on anyone diametrically opposed to the paladin's alignment (Good vs. evil, Lawful vs. chaotic, and neutral gods' followers chose to smite either good or evil).
3) Ranger replaced by the Woodsman from Wheel of Time. Bard modified to have no spells but get Bardic Music, Bardic Lore, and a 1/4 levels sneak attack progression. Assassin and other prestige classes that use magic altered to not use magic.
4) Expert Class -- The expert gets an average bonus HP progression and must choose a "theme" at first level. Then he selects ten skills as his class skill list, which must all center around his theme. This class is VERY useful for almost any kind of NPC, and it's a good PC class as it is the fastest way to get into the spell-casting PrCs. At six + INT skill points per level, it was relatively popular, too. Experts also got a skill similar to bardic lore, but could only use it on questions within their area of expertise.
5) Added a 1st-level only feat "Inherent Magical Talent" that was required for monks, druids, clerics, and wizards. (This helped balance monks in the low-magic world, costing them one feat.)
6) Druid, Cleric, and Wizard classes are PrCs. Each one requires 8 ranks in the appropriate Knowledge skill and the Inherent Magical Talent feat. Only those who start in the Expert Class can get all the spells (exception: A woodsman can become a druid with all the spells.) Without changing much, I just converted the existing classes to 15 level PrCs.
7) A caster gets a number of spell points equal to their caster level plus their modifier for the primary score. A first level spell costs one point, etc. Metamagic costs extra points, but does NOT change the level of the spell. (So a wizard only capable of 1st level spells can still use metamagic.) Also, magic save DCs were slightly bumped upward -- the save was given by 10 + spell level + primary atrribute modifier + secondary attribute modifiers, averaged. (Thus, for a wizard, it was 10 + spell level + Int mod + ((Cha. mod + Wis mod)/2) Not as complicated as it sounds since you only calculate it once and something of a bonus for the difficulty of learning magic in the first place).
8) Added the overchanneling system from Wheel of Time for when the caster was out of spell points. Each attempt to overchannel caused fatigue (subdual damage) equal to 2/level of the spell attempted. However, a caster with good skills could overchannel a LOT of low-level spells before he had too much subdual damage to continue.
9) All spells raised by one level -- cantrips become 1st level spells, etc. Any spell that does direct damage to the target is also increased by one level. Thus, fireball becomes a fifth level spell. Magic Missile was completely removed, in favor of "Chest Pains" which had a somewhat similar effect but allowed a saving throw. The highest level spells in the game are fifth level spells. (Keep in mind, though, that under the G'n'G rules, a fireball could decimate a level 20 warrior if the caster rolled average or better on the the damage.) A large number of low-level curses and hexes were added that had little to no combat effect but were a lot of fun to play with.
10) A magic-user can cast a level of spells equal to his number of ranks in the appropriate Knowledge skill divided by 4. Thus, an Expert5/Wizard1 (Wizard requires 8 ranks of Knowledge Arcana) is capable of casting a 2nd level spell, but only has one spellpoint -- so he has to overchannel to cast to the limit of his ability.
11) Spellcraft roll on casting a spell -- DC equal to the level of the spell squared + 10. Thus a first level spell is 11, whereas a fifth level spell is 35. (By the time the caster can handle a 5th level spell, however, that is about a 50% chance to successfully cast the spell.)
12) Casting classes received varied numbers of spells. Druids got three spells/level, clerics got two spells per level (one of which had to be a domain spell, until they had all from their domain for that level), and wizards only got one spell per level. All of the casters could learn more spells by observing them used and making a successful spellcraft check.
13) Potions based on Alchemy, rather than magic. High level alchemical recipes required truly bizarre ingredients (One I remember off the top of my head is the left-hand of a murdered woman -- bonus points if anyone knows THAT reference).
14) Finally, in keeping with the G'n'G system, I adopted a modified version of the Wheel of Time healing system -- healing a person converted the damage to subdual damage. Subdual damage could then heal normally. I also brought over the "Refresh" line of spells, that suppresses fatigue for a while -- you can ignore some amount of subdual damage, but when the spell duration is up you actually end up with MORE subdual damage than you started with. This proved to be quite a boon to my casters, since they could suppress their fatigue long enough to toss off a few extra spells before they collapsed from exhaustion.
----------------
It turned into a really fun campaign, mostly using other humans as villains. Magic was still very, very powerful -- but with the G'n'G system making it so hard to survive to high levels, it was also very, very rare.
And that's what *I* want when I talk about a low-magic campaign.
