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.

Badi Jones

Duplicate Content Cure is an extremely simple plugin that makes WordPress more Search Engine 
friendly (And therefore improving SEO). It does this by preventing search engine spiders/bots 
from accessing pages like Archives, Tags, and Categories, which tend to contain duplicated content 
from elsewhere on your site. This is achieved by adding nofollow, noindex to 
these problem pages.
Duplicate Content Cure doesn’t have a lot of features that other SEO plugins have, but 
it also is lightning fast by comparison.

Duplicate Content Cure

Duplicate content cure is a simple plugin that improves SEO by preventing search engines from indexing pages that contain duplicate content.

Noindex Password-Protected Posts vs Duplicate Content Cure