Ahmad Karim

This plugin will display related posts in your 404 page template. The related posts will be displayed based on the invalid URL user visited. It will check against the post types saved in the settings and will look for similar posts which match the URI.

404 Related Posts

This plugin will display related posts in your 404 page template.

Rana Mansoor Akbar Khan

Best Related Posts is a straight forward and easy to use plug in. It shows related posts for the post user is reading. It also add the first picture in post as thumbnail beside the related post.
It is better than other plugins and widgets as it is good for internal link building and overall pagerank. it Lets you to design the look of your related posts list. It lets to write your own HTML and gives you access to some tags (link, title, image etc)

Best Related Posts

Shows related posts with thumbnails. Allows you to design your own layout using simple interface. Good for SEO and reducing bounce rate.

404 Related Posts vs Best Related Posts