Eldritch Theurge from Complete Mage... if you consider Warlock an Arcane class...
Benefit: Casting a quickened spell is a free action. You can perform another action, even casting another spell, in the same round as you cast a quickened spell. You may cast only one quickened spell per round. A spell whose casting time is more than 1 full-round action cannot be quickened. A quickened spell uses up a spell slot four levels higher than the spellís actual level. Casting a quickened spell doesnít provoke an attack of opportunity.
Special: This feat canít be applied to any spell cast spontaneously (including sorcerer spells, bard spells, and cleric or druid spells cast spontaneously), since applying a metamagic feat to a spontaneously cast spell automatically increases the casting time to a full-round action.
Benefit: Choose a metamagic feat you know. You can apply the selected metamagic feat to any spontaneously cast spell without increasing the casting time.
Normal: Without this feat, metamagic spells with a normal casting time of 1 standard action take a full-round
action for spontaneous casters. Metamagic spells with a casting time of longer than 1 standard action take spontaneous
casters an extra full-round action to cast.