Andreas Münch

This is a simple Plugin that lets you set multiple passwords for your password protected posts and pages.
On posts/pages with password protection it will show an extra Metabox with a field to input additional passwords, one in each line.
Note that if you just changed a post/page to password protection you have to save once so that the extra field appears.
<h4>Expire passwords</h4>
You can also make passwords expire after x hours when being used. You can find the settings under Settings -> Multiple Post Passwords.
Note that the actual deletion of the passwords is triggered by a cronjob which is run every 30 minutes. So even if you set your expiry time to very short, it may still take 30 minutes until the password really expires.
Also note that the expiration only works for the additional passwords, not for the standard WordPress page/post password.
<h4>Using lots of passwords on one page</h4>
If you are using lots of passwords on one page and the password check takes a long time, you should activate the alternative password check in the settings to speed up the password check.

Multiple Post Passwords

Set multiple passwords for your protected pages so you can give them to different users.

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.

Multiple Post Passwords vs Password Passthrough