Franklin invented the busybody based on a similar design he'd seen in Europe. It's a set of three mirrors meant to be hung from a second floor window. The mirrors are angled so that when you look out you can see who's knocking at your front door - a difficult prospect in some Philadelphia neighbourhoods, where the buildings are high and narrow. Of course, with a little tweaking, the mirrors can be used to watch your neighbor's door as well - hence the name (though I'd have called them 'Nosy Old Bats' and marketed them very much at the over-70s market).
Busybodies are still used in Philadelphia today, probably because knowing who's at your front door is handy, and spying on your neighbours never really goes out of style.