The Google Auth API is open, but are there any sites (outside of Google) that currently support using it for two-factor authentication?
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That isn't something companies post to the open world. That's like asking who all are using RSA Tokens. Well, there are too many to list. The two of them are the same thing with different names. It's becoming more and more common for companies to use this method to add another layer of security. For example, the gaming industry is pushing it on its MMO customers to help prevent their account from being hacked.