I've used the libpam module for Google Authenticator[0] before, the difference being that it isn't dependant on the availability of a third-party service, but you have to be much more careful about losing your device. That said Authy do have pretty great uptime[1]<p>Nice for production servers, but becomes a an annoyance after the cool factor wears off for servers that don't really need it :) Installable on aptitude using libpam-google-authenticator (guide [2]).<p>[0]: <a href="https://code.google.com/p/google-authenticator/" rel="nofollow">https://code.google.com/p/google-authenticator/</a>
[1]: <a href="http://status.authy.com/" rel="nofollow">http://status.authy.com/</a>
[2]: <a href="https://scottlinux.com/2013/06/02/use-google-authenticator-for-two-factor-ssh-authentication-in-linux/" rel="nofollow">https://scottlinux.com/2013/06/02/use-google-authenticator-f...</a>