I've been checking out rating and recommendation systems in Drupal. The first recommendation system sneaked in with Apache Solr -- the "More Like This" module. Here's a screen shot of the recommendations it gave me on my most recent blog post. I tweaked it to show lots of results and use longer words to determine matching content, but aside from that, I haven't done much customization. The first two or three recommendations are great, but they break down pretty quickly after that.
Getting Solr running is a challenge if you do it from scratch, and if you let Acquia help you, I think there's a fee (I'm still in my 30-day free trial after giving up on becoming sysadmin enough to make my MacBook a live server with Solr running). Up next: Fivestar!