It isn't really possible to specify free shipping for certain items since shipping rules are based on weight or price, although there is a sort of work-around you can do to achieve it. It really only works, though, if free shipping is the only shipping rate you want shown for the item.
1. Assuming that you have weights entered for all of your items, set the weight of the item that will have free shipping to zero.
2. Create your free shipping rate, then make it only available when the total weight of the cart is zero.
