Alternative Spell Casting System
The spell point system dramatically expands the versatility of a spellcaster. Since she's no longer tied to using a specific number of spell slots, she can much more easily adapt to situations. In effect, spell points make all classes work more like the sorcerer, and make the sorcerer (or bard) work even more like the sorcerer. In general, spellcasters become more powerful—though they aren't capable of casting any spell they couldn't before, they are now capable of casting more high-level spells per day and more of whichever spells they need. If a 15th-level cleric needs to cast a dozen times during an adventure, he can do that (though not much else).
One balancing factor is the cost for casters to increase the damage dealt by their spells. This cost helps to maintain balance between spells of different level. If you didn't have to pay more for a 9d6 than for a 5d6 (a 3rd-level spell costing 5 spell points), then the 9d6 would cost barely more than half as much as a 9d6 (a 5th-level spell costing 9 spell points), even though both spells deal equal damage.
If you use this variant, consider adding other game elements that influence (or are influenced by) spell points. These might include magic items that grant (or cost) spell points, feats that grant bonus spell points (or make certain spells cost fewer spell points to cast), special abilities that drain spell points from casters, and so forth.