One of the related posts best hack I have ever known is posted by BloggingTips. If you are interested in related post for Blogger, I suggest check the article here.
The post explains that there are five methods to create a related post widget s for Blogger:
Related posts by Categories – List in Sidebar
Related Posts by Categories – List Beneath Post
Related Posts Widget – Beneath Posts
Related Posts Plugin for Blogger – Beneath Article
Feel free to try them folks :D