Consumers may look at it as an extra stocking stuffer, but for Wal-Mart's online rivals it could be the grinch who stole a lot of Christmas sales. According to the New York Times, Wal-mart is offering free shipping for many online purchases. Wal-Mart says it's serving penny-pinching consumers. But the free shipping -- which runs through Dec. 20 -- is a strike at Amazon, one of Wal-Mart's big online competitors.