This document compares several open source tools that can be used for data science. It provides background on key concepts in data science like data mining, machine learning, predictive analytics and business intelligence. It also discusses techniques commonly used by data scientists like clustering, classification, regression etc. The document then reviews popular open source data science tools like Orange, RapidMiner, KNIME, Weka and R and compares their key features based on techniques covered in the EMC Data Science Associate certification. It finds that these tools provide capabilities for common data science techniques at no cost, making them suitable alternatives to expensive proprietary software, especially for small organizations.