Posters are a great way to advertise because they get peoples attention and they are something different. You may be handed a free poster as an advertisement for an upcoming event in the area. You may want to hold onto it though as you never know what may someday become a valuable collectors item. If you need to advertise something to a large group you may want to offer them free posters.
Sometimes you will find a poster you really want, and if you ask for it they will give it to you. I recently went to a concert but I was at a movie theatre watching it. This was coming in on a live feed from New York. I really wanted the poster in the display case so I asked the manager. They told me after the show I could have it as I was the first one to ask about it. I was glad I asked early as when I went to get it after the show he said he had numerous requests for it throughout the show.
Magazines often offer free posters as well for people to tear out and keep. Most of the time they are featured in pin up magazines. Some types of sports magazines offer them as well. In order for the free posters to fit into the book they are folded in fourths. Be careful when you take them...