Since the list of onebox-supported sites seems to change over time, I was wondering if there is a list maintained somewhere?
onebox/lib/onebox/engine at master · discourse/onebox · GitHub has a list of all the built-in onebox providers.
However, since a few of those are generic providers, it’ll also work with most sites that have onebox-y displays in Twitter and Facebook.
It’s kind of the wrong question to ask, better question is, which websites are OpenGraph and/or oEmbed compliant? Lots of them are since that makes them work better in Facebook and Twitter when linked etc