Duck stamps that are active for the current year are worth $15, as of 2014. "Duck stamp" is an informal name for Federal Migratory Bird Hunting and Conservation Stamps. They are issued primarily as a yearly license to hunt migratory waterfowl, but they also allow bearers to enter wildlife refuges.
Some states also issue their own yearly version of the duck stamp. A duck stamp for the current year is only worth its face value, but stamps from previous years may be worth more than their face value to collectors if they are in good condition. The National Duck Stamp Collectors Society sells a wide range of older duck stamps directly and is a good starting point to get an idea of the value of an older duck stamp.