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http://en.wikipedia.org/wiki/Security_token
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That is a great idea. AOL used to give the RSA SecurID to their employees and the way it works is: the service sends a new one-time password to the token every 10 minutes or so, and you have to input that password along with your regular password. Very secure and affordable ($45 on the RSA website), and I'm sure there are inexpensive alternatives that work just as well.