In my campaign, sorcerers are wizards who have taken to studing that dodgy path of dealing with extraplanar entities for magic. They have just as much natural talent as wizards, but anyone can be a sorcerer. The magic system in my world works like this:
1) Paladins, rangers, crusaders, and vicars are the divine miracle-workers. Their powers aren't spells, but rather miracles straight from God. They still have to request them ahead of time via prayer, demonstrating foresight, so aside from a good-only restriction on rangers and what used to be clerics and druids, the system hasn't changed. Of course, wizardly trappings like wands and potions had to be done away with for flavor reasons, so for divine miracle-workers, Craft Wand was replaced with Craft Icon, Brew Potion was replaced with Water to Wine, etc.
2) Everything else is based on the planar cosmology of the setting. The Prime Material is surrounded by three other planes, the Spirit World (Astral Plane) that separates it from the Heavens, the Shadow World (Ethereal Plane) that separates it from the Hells, and the Pure Energy Plane, a dimension that overlaps the Prime Material and represents its nature magic and life force. Druid-type casters, like adepts and shugenja, as well as monks, draw on the Pure Energy Plane (think Jedi here). Psions pull mana, or ectoplasmic energy, off the Astral Plane to fuel their powers. Arcane casters pull ether, or quentissential energy, off the Ethereal Plane to power their spells.
3) Wizards and bards alike study arcane lore to learn their spells, but sorcerers take a more dangerous road. Their casting methods are the same, but sorcerers need no books because they're granted supernatural powers by big-shot elemental lords and outsiders, sort of like standard-setting clerics. A sorcerer has to be within one alignment step of his elemental patron, and he does better when casting spells from his patron's element or alignment but worse when casting opposing spells. For the elementals, I pretty much took them off of Final Fantasy, so my game has sorcerers of Bahamut, Ifrit, Shiva, Quetzalcoutal, etc. It seemed like a good idea, since I dropped the standard clerics, and so far, it works.