The information in the plugin information screen says
Do I need a key?
You need a key to embed "any" url, otherwise the plugin will only be available for these providers.
"these" then lists all the urls that embedly supports.
So I installed it, but when I tried to embed a youtube video it said I needed a key, but the information above clearly states that I don't need a key if I want to use one of their providers.
AM I doing something wrong to embed youtube videos? or Do you need to sign up and grab a key (I know they are free, but that's not the point).