So I was looking through Complete Mage yesterday and noticed the Retributive Spell feat. For a +1 slot adjustment, the modified spell stays dormant for up to 24 hours, during which you can spend an immediate action to have it go off on a creature that hits you in melee.
So, what are the most useful spells you can think of to use with this feat? The one thing that immediately occurred to me is that it gets around spells with long casting times. Geas, anyone?