KaeruCT

This plugin allows passwords for password-protected pages/posts to be passed directly through the URL.
The query string parameter that should contain the password is <code>pw</code>.
For example, if the URL of your post is <code>http://myblog.com/password-protected-page/</code> and the password is <code>PASSWORD</code>, 
then just append <code>?pw=PASSWORD</code> to it.
If the URL already contains a query string (for example, <code>http://myblog.com/?p=5</code>), then be sure to append <code>&pw=PASSWORD</code> instead.

Password Passthrough

This plugin allows passwords for password-protected pages/posts to be passed directly through the URL.

Julio Potier

You can now add passwords as many as you need on every protected page.
Why? Because you can protect page and give your client a personnal password, not just a random pasword or everyone.
Option: If you are logged as admin or the page’s author, you won’t need to enter password.
Option: If you set a list of child pages, just change any password (parent page or any child) and all the password for the hierarchy will be updated!

Multiple Passwords for Protected Pages

Add more than 1 password to your protected pages, also administrators and page's authors can view the protected pages without password.

Password Passthrough vs Multiple Passwords for Protected Pages