I'd heard them called bagworms, but they're not actually bagworms.
They're tent caterpillars, Malacosoma americana. They especially like our choke cherry trees - and there are tents in every one of them. They grow up to be moths. Here are a bunch of pics of the adult moth.
I think they're pretty with the sun shining through the tents. At least until they get full of frass [poop]. Then it's just disgusting.