There are quite a few articles around these days claiming to be a way to travel the world for free. Most of them seem to be methods of trying to build up points from airlines so that you can get free flights and other freebies.
To me, the best thing to do is to try and find organizations like WWOOF, which places people on farms to do work in exchange for food and shelter, or Couch-surfing, which organizes people around the world to lend a couch to each other when they need one. There are plenty of places that organize ride sharing as well.
It won't end up being completely free, but your experience will be much richer for having met more people.