Ben Doherty (Oomph, Inc)

Specify a list of tags to keep hidden: These tags will be invisible to users, 
such as in tag clouds or in post tags lists. This is useful when using tags to 
control behavior of your blog, or when you wish to maintain groupings of posts 
out of the public eye.
This does not prevent tag archives for hidden tags from being accessible, only 
hides tag links from tag lists and tag clouds.
Capable users (with the see_hidden_tags capability,) can see hidden tags in 
tag lists (but NOT tag clouds.) Hidden tags will receive the .hidden-tag class 
and are grayed out by default.

Oomph Hidden Tags

Hide certain tags from tag lists and tag clouds. Allow capable users to see hidden tags with the see_hidden_tags capability.

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]

Oomph Hidden Tags vs WPKeyMe