A quick google search shows a few; http://wordpress.org/extend/plugins/wordpress-23-related-posts-plugin/ http://wordpress.org/extend/plugins/yet-another-related-posts-plugin/ And some more metioned on matt cutts' blog; http://www.mattcutts.com/blog/wordpress-plugin-related-posts/
In my opinion, the Yet Another Related Posts Plugin is the best, because it can analyze title, body, categories, tags of posts to give a relation score. I've seen some other related plugins, but they just take some posts from the same categories or tags, which maybe in some cases less related.
I use WordPress Related Posts . I think it is one of the best out there you can use. Similar Post Plugin is very important on Wordpress SEO.
related posts plugin make have problems working properly! you may want to use YARP. checkout: http://corpocrat.com/2009/03/23/which-wordpress-plugins-to-show-related-posts/