Directed Edge (http://directededge.com) helps you find related stuff. It's a recommendations engine that plugs into your site to deliver Amazon-like recommendations. You can show your users personalized recommendations and similar content or products based on data you're already collecting. I implemented my own PHP bindings for Directed Edge's REST API. It makes use of cURL and DOMDocument(). The functions are abstract enough to plug and play with your own code, and is not dependent on any particular RDBMS.