Standard half board vouchers are valid in Chuck Wagon Café, La Cantina, The Grange at Billy Bob's, Plaza Gardens, Buzz's Pizza Planet, and Restaurant des Stars for an all you can eat buffet and one soft drink. They are also valid for whatever their current cash value is (I think it's €27 but might be wrong) at all restaurants in the parks and hotels and all Disney-operated restaurants in Disney Village (from memory again this is Annette's, the Steakhouse, and Café Mickey).
They are not valid on their own for "all the Disney hotels except the Disneyland Hotel" – Cape Cod, the Hunter's Grill, and the Parkside Diner need Plus vouchers.