There's a very nice cafe called Mar Adentro on Rua do Alecrim, between Chiado and Cais de Sodre metro stations. You can sit and have a coffee whilst plugging in your laptop to the supplied power and stay as long as you keep sipping. They don't have computers in there so you need to take your own. The staff are really nice and they really saved my bacon when I neeeded to finish some work.
There is another place near Chiado that has computers but I think you need to pay.
This tip has the address "Rua de Alecrim 35, Chiado, Lisbon"