I have double french doors on the south-face of my house, and I was wondering if solar air heaters built to fit the glass areas would be effective heaters.  I think I have seen these online before, people use them in the windows. I'm in Atlantic Canada, so I get good exposure from the low sun.

Other then redirecting the heat there is really no advantage to putting them inside a home. Once the sun is in, the heat is in, putting anything else on the inside of the window won't get any more sun in, but it can allow you to direct where the heat goes.

The only way to really add more heat is to add more windows or a solar air heater on the south side of the house.