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This powerful plugin makes it super easy to manually select related posts for your content. You can search and select posts that fit your criteria, all from within the write/edit post screen.

Creating a system of related items on your website will result in more page views per unique visitor, and will improve Google's understanding of your content.

As mentioned by WordPress SEO master Yoast:

A very good alternative is Microkid's related post plugin, which lets you manually pick related posts. This might cost a bit more time before you hit publish but might very well be worth your while.

You can display related posts automatically underneath your content, use a shortcode or use the widget. Or, if you have some development skills, you can use one of the available API functions for full control of the way related posts are displayed on your website.

With its full support for Custom Post Types, this plugin will work perfectly for both basic and advanced WordPress installations.

See the official plugin homepage for Related Posts for WordPress for more info and discussion.

Features

• Easily find posts or pages that might be related to the one you are writing with an integrated AJAX powered search utility
• Easily add and remove relations with a single click
• Extreme flexibility in the customization of the way the related posts are displayed on your website
• The relationship created works reciprocal, which means that if post A is added as related to post B, post B is automatically added as related to post A as well (this option can be turned off easily through the settings panel)
• Custom post types support
• Multiple widget support with extended customization options
• Shortcode support: add the [related-posts] shortcode to your post content to display related posts anywhere you want
• Automatically display a list of related posts underneath your posts content. No need to add any PHP tags in your template files, but several functions available for advanced users.
• Available in a growing number of languages