WPKeyMe

This plugin allows you to require a secret key that is passed via the URL: http://example.com/post-title/?key=[string]
Specify the key in a custom value called “key” with the [string] or use the built in widget.

Stable version of the plugin are pushed to the WordPress repository,
but the latest version of the plugin can be found at:
https://bitbucket.org/excion/wpkeyme/

Issues/Bugs: https://bitbucket.org/excion/wpkeyme/issues