They are an excellent source of antiques and collectibles. I even have a set of dishes that have been passed down from generation to generation, starting with my great grandmother, that originated from a garage sale. Some things have even proven to be quite valuable. Cookie jars are a good example. A few of my aunts have picked up some worth hundreds of dollars, paying only ten or fifteen.
Garage sales are also the perfect place to find books.
Books are one of the most common things to find at any garage sale. Nearly everyone owns a few and garage sales seem to be the best way to get rid of them. The best advice I can give to anyone who wants to go garage sale hunting is to be patient. Take your time going through everything at the sale. It may take going to several sales to find that special something, but it's usually worth the wait.