Magic Missile at will, you ask? I know an item which does that. It's called a Wand of Magic Missile!
This is exactly what I would recommend.
I am always very leery of any infinite use items, because they are doorways to methods of abuse you never imagined and can unintentionally change the entire feel of the game. A normal wand should have enough charges to be useful for the purposes that have been mentioned (running out of spells, etc) without encouraging players to find ways to break the world with their new toy.