To the OP: You might want to be a bit more specific about what *kind* of system you'd like. D20 abounds with variant spellcasting systems.
Some examples I've played:
1) Arcana Unearthed/Evolved. This is the system I hope gets used for 4e; it is both more consistent and more flexible than 3e's core spellcasting systems, and IMHO goes the right direction: Pulling out the very powerful "problem spells" (time stop, polymorph, etc.) and replacing the power they give with general casting flexibility on the part of the magic-using classes.
In brief, the AU/AE system divides spells into three categories (simple, complex, exotic) and multiple descriptors (fire, acid, [race], negative energy, etc.). Casters gain access to spells by both category and descriptor; they do not need to maintain spellbooks or choose spells known. A caster may have access to all simple and complex spells (the standard mage), all simple spells and all [simple, complex, AND exotic] spells with the plant and positive energy descriptors (the druidic healer type), and so on. Casters can take feats to broaden their spell access (for example, the Corrupt Mage feat allows you to treat all exotic negative energy spells as complex spells, and all complex negative energy spells as simple spells).
From their list of available spells, casters "ready" a set of spells, typically about 150-200% of the number of spells of each level that they can actually cast. They can then spontaneously cast any spell from their set of readied spells just as if they were a sorcerer casting a known spell. The major differences are as follows:
-Casters can apply metamagic feat-type modifiers without modifying the casting time. Moreover, instead of adding an effective level increase, modifiers require an extra spell slot of the same level as the spell being cast (called "lading" the spell).
-Casters can take feats that add a spell template (an elemental or energy effect, ability damage, extra duration, etc.) to the spell, either for free or by lading it.
The nice thing about this system is that it's very easy to build themed casters through judicious choice of feats that grant access to spells based on descriptor; in fact, even though there is no division between arcane and divine magic in AU/AE, I've found it very easy to build analogues to clerics and druids. Casters have much more flexibility, and because the problem spells are pulled out (and the artillery spells are generally a bit less powerful and ubiquitous) the system encourages a more creative use of magic.
2) The OGL Conan magic system. This is for DMs who want very little magic use in their games in general, and want it to be largely the province of NPCs. The Conan system uses a very select set of spells, most of which involve summoning demons and other spirits and which involve a personal risk to the summoner. Even the weakest of these spells (except for the personal augmentation effects, which IMHO are just sucky, since the class that can use them is super-weak in melee) are pretty powerful, but a scholar (the Conan spellcasting class) cannot cast more than a precious few of these per day. Spell DCs vs. saves are largely eschewed in favor of a Magic Attack Bonus system. The scholar gets decent class features (lotsa skill points, an okay set of hp and defense bonuses) to compensate for being unable to be the local artillery platform. I like this class, and the system in general, but it's not for people who want serious PC magic users.
3) Elements of Magic. IMHO, the Mythic Earth version is the way to go here; it's simple, it's clean, and it represents a genuine paradigm shift from the slots/points system. The traditions are also a good hook for generating themed casters (as you can see, a big asset for me!). Now if only my players would go for it...
4) True Sorcery. Okay, clean breast here; while it *is* flexible (even more than EoM:ME, AFAICT), I don't like this system. Too complicated, too fiddly, too wacky. But maybe someone else (like Henry; hint hint!) will come along and make some statements in its defense?
