Apple's MagSafe power cables are great. They quick release from their port and protect your notebook from clumsy adults and over-zealous children who get tangled in your power cord. The first generation of these cords, however, had a defect that caused the wires to fray; that posed a potential fire hazard to customers. The problem was widespread enough that customers with these early cables filed a class action lawsuit that Apple settled in November 2011.
Under the terms of the settlement, customers are eligible to receive a cash payment that covers some or all of the cost of a replacement cable. Those affected by this lawsuit have until March 21, 2012 to file a claim for this replacement cost. You can read about the terms of the settlement and a FAQ at a website dedicated to this case.