Navigo

The Carte Navigo is free (gratuite) for residents of Paris and Isle-de-France. The pass can be credited at ticket booths and machines for weekly (hebdomadaire) or monthly (mensuelle) use. If you don't have a French address you can still purchase a carte navigo découverte for 5 euros and then replenish it with weekly or monthly credits. You'll need a passport size photo to be attached to the card in order to make the purchase. Once you have it you can use it over and over again. If you're a regular traveler to Paris, consider getting one rather than buying the metro tickets. It is good on all public transportation. Above left is a photo of the Navigo-only turnstile that is in most of the métro stations entrances.