Paris Museum Pass covers most of the museums and attractions, including the Louvre Museum, Les Invalides, Chateau de Versailles, Centre Pompidou, Sainte Chapelle, Notre Dame Cathedral, Musee D’Orsay, Pantheon and Sacre Coeur. (Other popular attractions like the Eiffel Tower and the Catacombs are not covered). As we visited Paris during peak summer time, the use of the pass saved us a lot of time queuing to buy tickets at each individual attractions.
You can reserve it online and pick it up at the main office, but I thought it was easier to buy it at tourist information counter the airport. Just follow the signboard to get to the information counter.