While the pins themselves are not free, you can trade the pins you don't like or want with cast members for FREE! All Walt Disney World cast members wear a lanyard or have a pouch filled with various Disney pins. When guests ask to trade pins with them, they have to do so (as long as your pin is an official Disney pin and in good condition. See rules here). Also, if you are at the right place at the right time, you may find a guest who is a "Disney Pin Enthusiast" willing to trade as well!
Read more tips and tricks about Disney Pin Trading here.