michaeltyson

Specify a list of tags or categories to keep hidden: These will no longer appear anywhere 
on the site, such as in the tag cloud or on the list of post tags. This is useful when using 
tags to control behaviour of your blog with other software, or when you wish to maintain 
groupings of posts out of the public eye.

Hidden Tags

Hide certain tags/categories from the public

hauxh

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

WPKeyMe

This plugin allows you to require a secret key that is passed via the URL: http://example.com/post-title/?key=[string]

Hidden Tags vs WPKeyMe