This is a real insider's tip that they don't offer to tourist. By law because the French refused to carry a chip that could track their movements these cards are available to anyone. You can save money by purchasing a pass called "Navigo Découverte" (NAV-VEE-GO DEE-CO-VAIR)and have unlimited weekly or monthly Metro travel in Paris. You can buy it at any metro station or cigarette shop that sells tickets. The card cost 5 euros and is good for 10 years. Depending how many zones you need to travel in the price starts at less than 20 euros for central Paris where most of the attractions are. You can use the pass on the buses, metro or RER.

You MUST bring a photo copy of a passport or drivers licence size photo (black and white is OK) to insert in the pass then you can load it by credit card at multilingual kiosks. Compared to a 6 day museum pass that includes metro you can save almost 50 euros.

The Paris metro has lots of above ground track that goes through some of it's most interesting districts so having unlimited travel is nice.

Take note, if you are traveling mid-week, the weekly pass runs from Monday until Sunday so it may be cheaper to buy 10 tickets at a time but if you are staying for more than a week or like zipping around the Navigo Découverte might be for you.