Weka is a collection of machine learning algorithms for data mining tasks.
TLA-team: Weka is a collection of machine learning algorithms. It has tools for data pre-processing, classification, regression, clustering, association rules, and visualization. The algorithms can either be applied directly to a dataset (stored in ARFF(Attribute Relationship File Format) or called from one’s own Java code. RapidMiner ((http://rapid-i.com/content/view/181/196/) is also another option for a machine learning package. It was originally called YALE. It is becoming increasingly more popular. See here: (http://www.kdnuggets.com/polls/2010/data-mining-analytics-tools.html)