EDIT: Looks like they've changed this promotion to now require a purchase.
EDIT #2: I received a coupon in the mail for a free medium coffee, so I believe this offer is still available, but not advertised.
If you join Dunkin' Donuts Perks program and use the promo code DDPERKSCoffee, they'll send you a coupon for a free medium coffee. Chances are they'll also send out periodical coupons like most rewards clubs do as well. I should probably add this offer to our Customer Reward Programs list later today.
Wait a minute… woo hoo! My To-Do list now has over 100 items! Do I win a prize?