This is the (or an) official Tourism office in Marseille. Located adjacent to the Vieux Port, it's…read morea central location that has an extensive set of guides and tour books inside. Some of the local city tour companies have their starting points outside the office usually somewhere along the street. A helpful and necessary place to visit if you're looking for a map or someone who speaks English that can guide you wherever you need to go in the city.
The official city tour of Marseille run by NAP Tourism stops right outside the office. It's about a 90 minute tour or so with an audio track. You go from the old port up steep side streets to the Basilique Notre Dame where you have 40 minutes to go around, and take pictures. Then it goes around the rest of the city highlighting important districts, other prominent churches, and the like. It's NOT a hop-on, hop-off tour besides the one stop at the Notre Dame cathedral so it's nice to sit on a warm bus on a colder day.
You get a nice overview of the city and I encourage you save a day to take a cliche tour. It's especially good if you're only in the city for a short time and want to make the most of it.