Extra Slot( Complete Arcane, p. 79)
You can cast an additional spell.
Caster level 4th,
You gain one extra spell slot in your daily allotment, at any level up to one lower than the highest level of spell you can currently cast. For example, a 4th-level sorcerer (maximum spell level 2nd) gains either an extra 0-level or 1stlevel slot, and is able to cast any spell he knows of the chosen level one more time each day. Likewise, a 4th-level wizard can prepare any extra 0-level or 1st-level spell he knows. Once selected, the extra spell slot never changes level.
You can gain this feat multiple times. Each time, you gain an extra spell slot at any level up to one lower than the highest level of spell you can currently cast.