If you'd like to provide recommendations, suggest similar users, provide listings of similar products or categorize things, then Switchboard is a fast and easy to use service that handles all the math and AI for you. Just specfiy your relationships via the JSON API and Switchboard will have different types of recommendations ready for your users. Keep sending in your ongoing relationships to continuously improve and refine your recommendations.
Switchboard is currently under construction, so we've haven't fixed on pricing yet. To help with this, please do try out the beta when it's available - the more usage data we have, the easier it becomes to determine a fair price.