[stephengoodwin]

There's the exclave Point Roberts, Washington (http://ngm.nationalgeographic.com/ngm/0408/feature7/), which falls ever so slightly south of the 49th parallel.

[lisper]

I once visited Port Roberts just to see the border crossing. I drove in from the Canadian side, parked my car, and walked across the border into the U.S. without being challenged. No one asked for my passport. As far as I could tell, no one even noticed that I had crossed the border. But when I tried to walk back into Canada the Canadians gave me a pretty hard time and almost didn't let me back in. It was an interesting experience.

[haberman]

I've visited there too. Drove in both directions, seemed like not a big deal.

I heard that kids there go to school in Blaine, which means 4 international border crossings every day!