Versatile Spellcaster( Races of the Dragon, p. 101)
You can use two lower-level spell slots to cast a spell one level higher.
Ability to spontaneously cast spells,
You can use two spell slots of the same level to cast a spell you know that is one level higher. For example, a sorcerer with this feat can expend two 2nd-level spell slots to cast any 3rd-level spell he knows.