Google Authenticator is a plugin for self-hosted WordPress installs that lets you require Google’s Two Factor Authentication application for access to WordPress.
I’m currently using it on this blog and have to enter a numerical code from the Google Authenticator on my phone very time I login to the admin area. The setup was trivial and it has worked like a charm.
The only drawback is that this renders your blog inaccessible from programs that are not set up to prompt you for a Google Authenticator code, such as the WordPress mobile apps for iOS and Android.