Since the "Coca-cola" study there have been numerous experiments that confirm that when you give, you shall receive. This is great if you are giving in a principled manner, but this is where the slippery slope of ethics enters the equation.
Marketers use the principle to generate revenue. When you receive a letter in the mail asking for donations, it often comes with something for free. Years ago it was a small packet of stamps or address labels. Recently, I received a letter from a non-profit organization that contained a brand new, crisp, one dollar bill.
At a shopping mall if you go near the food court there is almost always someone handing out free bites of their most popular items. Go to a trade show and you will walk away with a bag full of "freebies" or "swag". Surf the Internet and inevitably you will be offered a free newsletter, samples, or a free trial.