We wanted to show related posts on our blog, so we started off using the built-in functions of WordPress. Then, we noticed that many of the posts that WordPress decided were related had absolutely nothing to do with anthing... So, we tried some plugins. There were plugins which used related and similar tags, some which used categories, but they all seemed to be very mechanical. The lists they generated were all obviously computer generated. Then, we had an idea... why don't we have the option to decide for ourselves which posts really are related? Several lines of code later, the Related Posts Picker plugin was born! WordPress will give you a list of posts that it thinks are related to the one you're updating, and you are able to tell WordPress which posts really are related. You can configure the number of posts you have to choose from, and the maximum number of automatically selected posts that will show if you don't choose any.
So, go check it out, and if you find it useful, then download it here.
To use the plugin in your theme, simply add the function: wherever you would like your list to be shown.
You will need to tag your posts and save them before it can come up with a good list of options to choose from.