Duo provides simple two-factor authentication as a service via:

    1.  Phone callback
    2.  SMS-delivered one-time passcodes
    3.  Duo mobile app to generate one-time passcodes
    4.  Duo mobile app for smartphone push authentication
    5.  Duo hardware token to generate one-time passcodes

This package allows an admin (or ordinary user) to quickly add Duo
authentication to any Unix login without setting up secondary user
accounts, directory synchronization, servers, or hardware.

WWW: http://duosecurity.com