This plugin returns a list of related entries based on a unique algorithm for display on your blog and RSS feeds
The Dynamic Related Posts Plugin (drpp) gives you a list of posts and/or pages related to the current entry, introducing the reader to other relevant content on your site. Key features include:
An advanced and versatile algorithm: Using a customizable algorithm considering post titles, content, tags, and categories, drpp calculates a “match score” for each pair of posts on your blog. You choose the threshold limit for relevance and you get more related posts if there are more related posts and less if there are less.
Templating: New in 1.0!
Caching: Improved in 1.0! drpp organically caches the related posts data as your site is visited, greatly improving performance.
Related posts in RSS feeds: Display related posts in your RSS and Atom feeds with custom display options.
Disallowing certain tags or categories: You can choose certain tags or categories as disallowed, meaning any page or post with such tags or categories will not be served up by the plugin.
Related posts and pages: Puts you in control of pulling up related posts, pages, or both.
This plugin requires MySQL 4.1 or greater.
Plugin homepage: http://111waystomakemoney.com/dynamic-related-posts/