Advanced d20 Magic

Advanced d20 Magic is an alternative spellcasting system for the "Dungeons & Dragons" role-playing game and the d20 System role-playing game. It is published by Guardians of Order, the producers of the Big Eyes Small Mouth game and the Slayers d20 game. The system presents a DC-based Magic System similar to the Sovereign Stone game published by Sovereign Press.
Dynamic spellcasting.
"Advanced d20 Magic" introduces a new spellcasting system called "Dynamic Spellcasting" to the d20 System. Dynamic Spellcasting is an alternative system to the standard "Vancian" System that was based on the magic system in Jack Vance's "Dying Earth" series. Instead of memorizing and preparing spells, a spellcaster can cast any spell he or she knows as a standard action without any special ingredients or other actions. In effect, a spellcaster can cast any spell he or she knows as often as she desires. 
This is balanced by the fact that a spellcaster must save against spell drain. In "BESM d20", this is represented by the fact that a spell drains energy points. In a regular Fantasy d20 game, this is represented by the spellcaster inflicting non-lethal damage on himself or herself.
DC based spells.
In "Advanced d20 Magic" each spell in the SRD is assigned a Difficulty Class. This Difficulty Class is nominally based on the Wizard class in the Dungeons & Dragons game, and the base difficulty class ranges from 10 for 0-level spells, to 51 for 9th level spells. These DCs are further modified by other properties of the spell—whether or not if they require time to cast the spell or material components in the regular Vancian system.
DC based item creation.
Also, in "Advanced d20 Magic" is a DC based item creation system. Also dynamically different from the standard Item Creation Rules in the d20 system, a Spellcaster isn't limited by what magic spell to use or what level to determine a spell's potency. Instead, he or she can use any similar spell such as "Fireball", "Flame Blade", or "Flaming Sphere" to create a flaming sword. The player just needs to create the item, the gamemaster needs to set a difficulty, and the player needs to roll a fortitude save against the difficulty class. It's that simple.
Spells already converted.
There is a large number of spells already converted to the Advanced d20 Magic system. These come out of the SRD. Every spell that is in the Dungeons and Dragons Player's Handbook is already converted. Also included in the system is a spell conversion system so a Gamemaster can convert his favorite spells from other resources.
Criticisms against dynamic spellcasting.
Critics cite the heavy use of mathematics involved in the Dynamic Spellcasting mechanics. A player needs to do some hefty calculations which could slow game play, because there are so many options for modifying the fortitude saving throw against spell drain. For instance, a fifth level Wizard in order to cast "Fireball" (DC 31) would have a +5 spellcasting modifier. Plus he can cast it as an Invocation (+5), an Evocation (+5), and add expendable foci (+2) = which brings the DC down to 14. And the mage can bring it down by adding other options. Such math can slow down game play for some players.