This does exactly that and you don't even create an account, so it's really anonymous: http://www.gosoapbox.com/ http://www.gosoapbox.com/ . All you have to get

This does exactly that and you don't even create an account, so it's really anonymous: http://www.gosoapbox.com/ http://www.gosoapbox.com/ . All you have to get is a shared url for the dicussion. Their primary use case is for classroom discussion, but functionality is what you described.