They offer one day admission tickets to the Louvre, Versailles Palace, and Musée d'Orsay. The date is open on the ticket. The price is the same as you would buy it at the entrance of the museum/monument. That means you can buy them in advance without standing in line at any of these monuments. They also sell museum passes for 2, 4 or 6 days (32?, 48? and 64?) . It is true that you can get tickets in advance from other places, like Fnac, Virgin Megastore etc, but they usually charge you extra!!!!
Once you buy the louvre ticket, you can enter the musuem through "Passage Richelieu" on the Rivoli street, which is about a 3-min walk from the tabacco store. It is a special entrance for visitors with tickets.

