There are many ways to add Drupal related content blocks on your website. One easy way is to install the new featured content module:

http://drupal.org/project/featured_content

You can create very useful related content blocks in just minutes. Some examples include:

• Related content block that shows the titles of the top 5 nodes with terms that most closely match the node being viewed and are of the content type "blog" and were authored by the same author as the node being viewed. If the page being viewed was created by user "Joe" and has terms "dog", "cat", "mouse", "pig", then the block will show up to 5 blog pages where the author is "Joe" and will be ordered based on how many of the matching terms the blog page has.
  
  
  
  
• Related content block that shows an ordered list of the titles of the top 20 most popular nodes where the first part/section of the node's path matches the first part of the path of the page being viewed. If the page being viewed is /groups/topic/1, then the top 20 most popular nodes with an alias like /groups/* would be shown.
  
  
  
  
• Related content block that only shows up if the node being viewed has the content type "story" and shows a random list of 3 teasers of nodes where the content type is "page" or "blog" and has the term "featured".
  
  
  
  
  

So, if you want to easily create useful and relevant Drupal related content blocks, check it out.