NOTE: This tutorial assumes you're using an extension that uses the "special_shipping_group" attribute... But you can also use this tutorial to output any attribute you'd like into the product view and product listing.
Ok... Say you're using the "special_shipping_group" attribute, to specify which products should receive free shipping.
You would, for example, add an attribute called: FREE SHIPPING
Make sure you also set this visible under product listing, and product view, or else it won't display.
Following, you will assign this attribute to all the products that are to use this "FREE SHIPPING" attribute.