The Paris Métro is among the great transportation bargains of the world: 400 stations all over the city, and you can travel from one to another for 1.40€.

If, instead of buying one or two tickets at a time, you ask the ticket-seller for a carnet, you'll receive 10 tickets good on the Paris Métro, city buses, funiculars (such as the Funiculaire du Montmartre that climbs the hill of Montmartre), trams, and even for short trips on the RER suburban trains, and you'll pay only 1.14€—a discount of 18%.

Other savings options, such as the Paris Visite and Navigo electronic card transit passes, may be useful for some visitors, but for most visitors to Paris, nothing beats the carnet.

(By the way, the Carte Orange weekly and monthly transit passes have been discontinued, and replaced by the Navigo pass.)