Once a year, the French government sponsors a weekend called the Journées du Patrimoine, during which places that are normally closed to the public open their doors and conduct tours. This weekend marks the Journées du Patrimoine for 2009 (the weekend is usually around this time of year). About 15,000 different places have opened their doors this weekend for people interested in seeing them. The most popular is probably the Élysée Palace, where the French president lives—the lines are typically hours long to get in for a tour, starting early in the morning. There are plenty of other places that are less crowded, such as subway repair centers, or historical monuments that may already be open to the public but without guided tours under normal conditions … such as La Coupole, a huge and popular restaurant near Montparnasse that dates from the golden years of the Montparnasse area, when it was one of the major hot spots in Paris (the area still popular now, but with fewer socialites and no gangsters).
I haven't gone out, since I'm not that interested in standing in line for anything, but overall this event is always a great success.