The Join Us in France podcast directed me to the UNESCO website. It looks like a good resource for finding interesting or important sites but which may not be well known. France has many such sites (you can probably name a bunch off the top of your head), but I found several sites that I never heard of. Here are few French ones of the man-made variety that hold my curiosity.
Fortifications of Vauban.
Belfries of Belgium and France
Vezelay Church and Hill