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I guess it depends how you define "walled garden", but HipChat is built on top of XMPP: http://help.hipchat.com/knowledgebase/articles/64377-xmpp-ja.... Facebook chat used to be as well, though I'm not sure if it is these days.

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If it where no walled garden, then I could reach the chat rooms at conf.hipchat.com from e.g. my gmail account. As the linked page says "Our server is not federated and will not let you send or receive messages with users on other networks."

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