The Lisboa Card is the tourist card of Lisboa, Portugal and it is always discussed if buying it is sensible. It is available for 24/48/72 hours at a price of 19/32/40 Euros. You can use public transport (including elevators and trains) as often as you like, some sights can be accessed free of charge and at other locations you get discounts of most often 20 percent.
We took the Lisboa Card for 72 hours and if I calculate all fees for sights and public transport we could have got everything for a slightly lower price. But as we were traveling in a very tight schedule I have the feeling that buying the card was right anyway: the feeling of being able to simply jump in every mean of public transport and already having the right ticket was worth the price. Just enter the vehicle in the front, hold the Lisboa Card in front of the validation machine and wait for green light.
You have to write date and time on the front side of the card and sign it on the back side. You’ll get a thick leaflet telling you what you can do for free and where you get discounts. You only have to carry it with you if you want to use the (shopping) vouchers in the rear part. The Lisboa Card is available in the arrival hall of the airport or at different hotels throughout Lisboa.