MOA is the most popular open source framework for data stream mining, with a very active growing community.
This resource provides a demonstration of the capability for Machine Learning algorithms to provide a measure of understandability given the output from the algorithm.
WEKA (Waikato Environment for Knowledge Analysis) is an open source machine learning software in JAVA. It contains tools for data preparation, classification, regression, clustering, association rules mining, and visualization.
The Advanced Data Mining And Machine Learning System (ADAMS) is a flexible workflow engine aimed at quickly building and maintaining data-driven, reactive workflows, easily integrated into business processes, released under GPLv3.