Paris, France and the RATP
Paris has a great public transit system that is ideal to shuttle tourists to the sites. Depending on where your hotel is, you may be able to walk to many of the sites, but some of them areas are a bit of a hike. Using the Metro is an easy way to get around. The map of the transit system can be found at the RATP website, http://www.ratp.fr/. There is an interactive map, and a very handy feature that will allow you to enter two addresses, and it will tell you how to get between them. But the question is, what ticket to purchase?
There are a number of options for tickets to purchase. The key options for a tourist seem to be the Carte Orange Hebdo (Weekly), the Carnet (individual tickets in pack of 10) and the Mobilis (unlimted day pass).
There is a tourist pass, but it is more expensive and more limited than the other options. It does provide you the ability to travel further outside Paris on the RER lines (outside of zone 1 and 2). In general, though, unless you are travelling outside of zones 1 and 2 (the core of Paris) a lot in a short period of time (3 days or less), then I can't figure out how this pass is worth while. Even then, a Carte Orange may be cheaper.
Because I am both a math and transit geek, and because I needed to figure out for myself due to a few demi-weeks I am spending in Paris, here's what I determined.
The Carte Orange gives you unlimited weekly trips (they have a monthly version as well). The Carte Orange is good if you are going to be taking 14 or more trips total in the period it is good (Monday - Sunday), and those trips will be on 3 or more different days. In that case, the 15.70 Euros (for zones 1 and 2) is the best deal.
Note that you do need a picture for the Carte Orange. A passport sized picture from a photo booth is ideal. If you don't have one, there are photo booths in the system. Chatelet in the heart of the tourist area has one, which for people in the core is probably your best bet.
The Carnet (10 tickets for 10.70 Euro) and individual tickets (1.40 Euro) are best if you are planning on taking less than 14 trips across 3 or more days. In this case, the total cost will be somewhere less than the 15.70 of the Carte Orange and will give you flexibility to travel on many days.
If you are going to be taking 5 or more trips in a day, however, you are best getting the Mobilis, which is a one-day unlimited pass for 5.40 Euros. If you do all your out of walking area site-seeing in two days, then you can spend 10.80 Euros on the Mobilis take 10 or more trips and come out on top.
You can, of course, combine the Mobilis for a heavy travel day and then individual tickets or a Carnet for the other days.
Finally, there are some options to get passes for the museums plus transit. However, I don't know how much museum entries cost, so I can't indicate if they are a good deal or not.
Enjoy the city of lights!