Kyle Gilman

This plugin does one thing and one thing only. It adds the text <code><meta name="robots" content="noindex"></code> to the header of any password-protected post. Google and all other respectable search engines will leave your password-protected pages out of their search results.

Noindex Password-Protected Posts

Disables search-engine indexing for all password-protected posts.

jonkemp

Adds the noindex meta tag to your WordPress Login page.
If your WordPress blog has a public login page, you probably don’t want it to be indexed by search engines. Noindex Login adds the robots “noindex” meta tag to your login page to keep search engines from indexing it. It also works on your registration and lost password pages.

Noindex Login

Adds the noindex meta tag to your WordPress Login page. If your WordPress blog has a public login page, you probably don't want it to be indexed …

Noindex Password-Protected Posts vs Noindex Login