Google-authenticator
From Attie's Wiki
Installation
Note: Installing this has a side effect - all users must have google-authenticator setup to login using a password
apt-get install libpam-google-authenticator echo "auth required pam_google_authenticator.so" >>/etc/pam.d/sshd sed -re 's/(ChallengeResponseAuthentication) +no/\1 yes/g' -i /etc/ssh/sshd_config
User Setup
google-authenticator