The Yubikey module for Drupal provides YubiKey strong two-factor user authentication capabilities for Drupal. The module is flexible and has several capabilities. Please note: not written by Yubico.
Drupal YubiKey Module
How to enable YubiKey + Drupal
To add the YubiKey two-factor authentication to Drupal, follow the instructions listed in the Yubico Wiki entry:
Please note: not written by Yubico