I have upgraded and consolidated my old Contextual Related Posts Plugin for WordPress. This new version (1.1) is a simple install (just install the plugin file) and it will show the last 5 related posts from your blog. The display show the links right after the comment form. The matching is done with the post title of the post being viewed. Examples include my Green Car Blog (wordpress 2.0) and Desi Dalal (WordPress 1.5)
PS: Some theme authors tend to remove default actions from their themes (Rin is a good example). If you are having problems displaying the related posts, please check with your theme author or post a comment here for help.
Download Contextual Related Posts Plugin
Download the updated version of Contextual Related Posts.