alvego

Simple and easy WordPress plugin for protecting your login form.
Plugin implements setting the security key for the login form.
Thereby significantly reducing the likelihood of automatic brute force.
The security key is transmitted manually as GET parameter for login form.
For example: http://example.com/wp-login.php?sk=my_secure_key
Warning: Be careful if <code>sk</code> parameter is not specified, the login will not be possible, even if specifying the correct username and password.
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Protected wp-login

This plugin protects your admin's login page (secure key in url).

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]

Protected wp-login vs WPKeyMe