Check out Arcana Evolved.
It's flexible, powerful, balanced, and unified. There's no arcane/divine distinction; instead, there's a simple/complex/exotic distinction.
Spells can be cast a level higher or a level lower to improve options.
Low-level slots can be combined to cast high-level spells and vice-versa.
"Templates" can be gained with feats to modify spells and add or remove effects.
Overall, it's complex but a hell of a lot of fun.
Although I haven't tried it in play yet, the Blue Rose/True20 spellcasting system looks pretty impressive, too. It's skill-based rather than spell-based.
-O