Collaborative Filtering Plugin for Rails

Also, be sure to checkout Mahout: a scalable, machine learning library built on top of Hadoop - http://mahout.apache.org/

Of course, if you really wanna scale it up and mess around with Hadoop, Cloudera's got some nifty resources as well - http://www.cloudera.com/hadoop/

Also, check out cwninja's branch - probably more up to date: http://github.com/cwninja/has_related