Guided tours in Montreal are offered on boats, buses, bicycles, and on foot. And in Old Montreal, you can still take a tour in a horse-drawn "calèche". Whether your interest is history, architecture, culture, or food, there is a tour for you.
Gray Line offers "Hop-On Hop-Off" double-decker bus tours, which is a good way to see a lot of sights in one day. They usually start at the Square Dorchester (Info-Touriste office).
Boat tours, from brunch and dinner cruises, to the “Amphibus” (a sort of, well, bus that also floats) start at the old port in old Montreal. For more complete information, ask Info-Touriste at Dorchester Square downtown.
More specialized guided tours are available. Heritage Montreal offers their "Architectours" for those who are interested in local architecture and L'Autre Montréal offers tours for those interested in urban issues - planning, history, ethnic and working class communities. Free Montreal Tours (although not actually free) offer group and private tours as well as neighbourhood food and beer tasting tours.