Google (obviously), Dropbox and AWS all support Google Authenticator, although Google Authenticator is just one client you can use on the client side. On the s

Google (obviously), Dropbox and AWS all support Google Authenticator, although Google Authenticator is just one client you can use on the client side. On the server side, one reason to run this in house is not having to depend on the availability and security of a 3rd party for something as fundamental as logging in.