You can use Google Authenticator to do two-factor authentication with LastPass, WordPress, Amazon Web Services, Drupal, and DreamHost, or even use a YubiKey device. There’s even a Pluggable Authentication Module (PAM) so you can add two-factor authentication to any PAM-enabled application. That means you can use Google Authenticator to add two-factor authentication to SSH, for example.
It’s pretty neat that the Google Authenticator app works without a network connection.
Update (2012-09-21): Rich Siegel reminds us to unregister any authenticator apps before upgrading to a new phone.