Purchase a Starbucks Card (the minimum amount is $5) and register it at the site. Starbucks will mail a free drink coupon to you. Note that if you have a depleted Starbucks Card, you could register it too.

“Limit one per customer. Two per household. Must join between June 3–July 14. Your coupon for a free beverage will arrive in the mail within 2 weeks (U.S.) or 3 weeks (CAN). Offer expires July 14, 2008. Good for one handcrafted beverage per person with coupon. Offer good only at participating Starbucks stores in the United States and Canada. Void where prohibited, taxed or restricted. Cannot be combined with other offers.”

http://www.starbucks.com/cardrewards/