The Paris Metro is one of the most remarkable man-made creations on earth. Chances are good that whatever point in Paris you wish to visit, it isn't too far from a Metro station. For that reason, it is fairly economical to get around this huge city.
Don't confuse the Metro trains with the RER train system. Both operate as subways in many places, but RER is more of a regional train network that takes you to outlying areas. There are numerous transfer points between the two systems. Be warned: Confusing the two systems is among the most common and time-consuming mistakes made in Paris by novice travelers.
Experienced Paris travelers might have purchased a "Carte Orange" that allowed unlimited travel at a modest price. As of May 2008, it no longer exists.
The Navigo Découverte has replaced Carte Orange, and can be purchased for 5 EUR ($7.80 USD) at any station. Once you have the card, you purchase time that is put on the card electronically. The card itself can be used and reused. For 17 EUR ($26.50 USD), you can have unlimited use of all but the TGV trains within zones 1-2. Do not confuse this with its cousin, the Navigo Card, which is only available to local residents.
Another pass to consider is the ParisVisite, available for a variety of zones and days. For example, a three-day pass for zones 1-3 is 19 EUR ($30 USD). Those zones cover a lot of attractions, but not Disneyland, Versailles or the airports. The pass also provides access to RER trains, buses and even the funicular that takes you up Montmartre to Sacre Coeur and a spectacular view of Paris. You can buy it in advance of your visit. The clock doesn't start ticking toward expiration until you actually use it on the system.
If you use mass transit, plan your day so you avoid rush hours in the morning and afternoon. Delays and frustrations are common.
Taxi stands in Paris are marked by a blue-and-white "Taxi" sign. You'll pay cash and you'll pay quite a bit, especially late at night or early in the morning. But don't hesitate to use a taxi if it will save you large amounts of time or if you think your safety is at stake.
The cheapest transportation to Paris' distant airports is the RER system. There are airport buses that leave from Charles de Gaulle Station in central Paris, but they cater to huge crowds of people with loads of luggage in tow.
Taxi fares can run as much as 40 EUR ($63 USD) between central Paris and the airports. That makes them a last resort for the budget traveler.
Also avoid car rentals unless you're going to tour the French countryside. With the cheap transit options available, you do not want to drive a car in Paris traffic.