Update: This coupon is actually for a FREE White Milk Gallon with any $100 grocery purchase now. Sorry about that! I rarely spend that much at the grocery store at once, but for those who do it's a nice little bonus!
You can print a coupon for a FREE half gallon of chocolate milk when you buy one gallon of milk again, by entering The Great Gallon Drive sweepstakes. You will need to click the white milk button at the bottom of the page after entering the sweepstakes. (Make sure to have your pop-up blocker off, or you won't be able to print your coupon.) This is a bricks coupon, so you can hit the back button on your button to print two. This coupon is limited to the first 125,000 people who enter and expires in one month.