Ariana

This is a very simple plugin that adds a “robots” meta tag to the head of your site with “noindex” in it.
The effect of the noindex robots meta tag is that search engines will not index your pages and posts at all. 
This is a better solution then the wordpress default settings option, which tells search engines to not crawl the URLs in the robots.txt. Because Google (and other search engines) sometimes still index URLs they haven’t crawled, for which they determine relevancy based on anchor text of links which lead to the specific URL.

Simple Noindex

radgh

This simple and lightweight plugin adds a “Hide from search engines” checkbox above the publish box to pages. By 
ticking this box, a meta tag will be placed into the <code><head></code> section of your page specifying that robots should 
not index the page.
By default this only applies to pages. You may extend this functionality to posts or other custom post types with a small bit of code. See the FAQ’s tab for further instructions.

Noindex Pages

Ask search engines not to index individual pages by checking an option in the publish post box.

Simple Noindex vs Noindex Pages