You need to "like" the Tim Hortons' US Facebook page to get the coupon, which you can find here. The deal is similar to one by Krispy Kreme, except the Krispy Kreme deal requires neither a coupon nor a purchase.
Tim Hortons is also offering two new, limited-time Timbits donut hole flavors this summer: Orange Tangerine and Caramel Glazed. They're offered in a 10-pack for $1.99.
The donut chain is also offering a $1 drink special on small sizes of their Frozen Raspberry Lemonade, Iced Coffee, and Frozen Lemonade.