In celebration of National Donut Day (it exists!), Dunkin Donuts is giving away one free donut with any beverage purchase on Friday, June 4.
Also, Krispy Kreme will be giving outof any variety with no purchase necessary. They have a list of their site with all of the participating stores, so it might be worth checking that out before leaving home.
EDIT: Hiffer Richard wrote in that Lamar's Donuts are also offering a free original glazed donut tomorrow. Score!