This list contains most of the non-SaaS recommender systems that are open-source.
What is Apache Mahout? The Apache Mahout™ project's goal is to build an environment for quickly creating scalable performant machine learning applications.
An open source and platform-agnostic machine learning deployment platform that helps data scientists add intelligence to organisations.
LensKit is free and open-source software, available under the terms of the GNU Lesser General Public license version 2.1 or later.
With the open source recommendation engine easyrec you can add recommendations to your website. easyrec is a Java web application providing a REST-API.