Yes, there are several open-source tools and frameworks available for building next-best-action systems, including:

• Apache Mahout: A machine learning library that provides algorithms for collaborative filtering and recommendation systems
• TensorFlow: An open-source machine learning framework for building custom models for next-best-action systems
• Scikit-learn: A library for building traditional machine learning models, including classification and regression, to predict next best actions

These open-source tools can be integrated with Snowplow's event data pipeline to power the next-best-action models.