there are indeed, and the barcelona transport system is well priced and works brilliantly.
i don't think it extends as far as the circuit, but if you buy a 10 journey ticket two people can use it at once, and you're allowed a very reasonable amount of time for connections. works out at about £1.10 or something per journey. it used to include the train from the airport as well, not sure it does any more if the metro is finally open though.
a car is handy if you end up staying near the circuit rather than in the city (eg mollet del valles), but if you're planning to enjoy the city as well then public transport is the best choice for sure