fotype

Adds a tokenized text box for saving comma separated values.
<h4>Compatibility</h4>
This ACF field type is compatible with: 
* ACF 5 
* ACF 4

Advanced Custom Fields: Token Field

Adds a tokenized text box for saving comma separated values.

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]

Advanced Custom Fields: Token Field vs WPKeyMe