The Haskell Free Library and Opera House in Derby Line, VT and Stanstead, Quebec. It was deliberately built (1904) half in each town. See the concrete marker near the flower pot? That’s the USA/Canada border. Residents from both towns are allowed in the library without going through customs, but if they go anywhere else in town, they must go through customs.
The Haskell Free Library and Opera House is listed in the National Register of Historic Places. Read its history! And yes, you can stand in two countries at once in this library. (Of course I did.)