I use the one that used to be at
http://www.w-a-s-a-b-i.com/archives/...ed-entries-20/ , but isn't anymore. I don't recall now where I picked mine up, since that location was gone before I tried to find it.
It works fairly well, although if there really aren't any related posts it'll still return the number of posts you wanted. It works best when you're writing articles in a series, it'll pickup the other articles in the series as related.
Jay