Elemental Adept( Complete Mage, p. 42)
You can spontaneously cast a spell of the element you have mastered.
Elemental mastery class feature (see the wu jen class in Complete Arcane),
Choose one spell of the element you have chosen for your elemental mastery class feature. You can now spontaneously cast that spell by sacrificing a prepared spell of equal or higher level. The spell you choose must be in your spellbook. When you gain a level, you can change the spell you can spontaneously cast by picking a new spell of the element you chose for elemental mastery.