The Haskell Free Library and Opera House is Stanstead's one truly remarkable sight. Built in the early twentieth century for communities on both sides of the Canada-United States border, it deliberately straddles Quebec and Vermont. Inside, the international line is marked on the floor; the opera house stage is in Canada and much of the seating is in the United States.
It is still a working library and cultural venue, not just a curiosity. Cross-border visits and access rules can change, so check the official site before you go and enter only as directed.