Bacalhoeiro, Rua dos Sapateiros 224, Baixa. Getting here can be a bit of a chore since the minor street where the resto is at doesn't appear on the map and the people in the tourist booths do not even know where it is. Rua dos Sapateiros is the street immediately to the right of Rua Augusta when you're facing the sea. Once here, savor some of Portugal's most authentic dishes (I got grilled squid and bread soup -- Y.U.M.M.Y!). Locals eat here and the food is insanely cheap and delicious.
Casa da India, Rua do Loreto 49-51, Bairro Alto. Their barbecued chicken is a must-have! It's nice, plump, juicy and cheap. The area where the restaurant is at is a popular place for Lisbon's nightlife.
Clube do Fado, Rua de São João da Praça 95 (beside the Cathedral). A bit pricey, considering that you can have a binge for 9 euros, but it's worth it if you're into fado. Try their fish of the day.