I just releasaed this Bridgetownrb static site builder plugin which leverages the power of TF-IDF (Term Frequency-Inverse Document Frequency) and cosine similarity to automatically add related posts to your blog entries. I'm considering doing something similar for Rails and Django. Seems to work well - the bigger the site the better the results.

Here's the Github repo https://github.com/mpclarkson/bridgetown-related-posts/