If you are a fan of the Related Posts plugin, then I think you will like the Similar Posts WordPress plugin. The Similar Posts plugin is based on the Related Posts plugin, but with extra functionality. Rob Marsh, the developer, even used a couple of my ideas (excluding categories and unrelated pages) in his plugin.
But Rob didn't just stop there, he improved my hacks by placing the "exclude pages and categories" options in the admin menu. No more messing with the code. I was actually planning on doing the same thing, just not sure when. Now I don't have to write my own plugin. Rob already did it.
Another nice feature of the Similar Posts plugin is that you can tweak the similar keywords for each post when you are writing the post. This gives you the ability to fine-tune the keywords to your liking. In addition, if you have previously used the Related Posts plugin to assign keywords to posts you can now import them all from the Options page. I didn't even know you could assign keywords with the Related Posts plugin.
Here's my little comparison of the two:
|Related Posts||Similar Posts|
|Can set the number of posts to show||yes||yes|
|Customize before/after post||yes||yes|
|Customize before/after excerpt||yes||yes|
|Show password protected posts||yes||yes|
|Exclude static pages||no||yes|
|Exclude certain categories||no||yes|
|Checks title for match||yes||yes|
|Checks content for match||no||yes|
The plugin is great, but I would suggest changing the "no similar posts" result when there's no related posts. Instead, it would be nice if the plugin could display a set of user-defined posts when the plugin can't find a related post. I have not seen "no related posts" on my site recently, but it will eventually show up again when I write an oddball post.