Weka is a collection of machine learning algorithms for data mining tasks. The algorithms can either be applied directly to a dataset or called from your own Java code. Weka contains tools for data pre-processing, classification, regression, clustering, association rules, and visualization. It is also well-suited for developing new machine learning schemes.

You can access the visualizations from the ClassifierPanel, ClusterPanel and AttributeSelection panel using the pop-up menu. Click the right mouse button over an entry in the Result list to bring up the menu. You will be presented with options for viewing or saving the text output and -- depending on the scheme -- further options for visualizing errors, clusters, trees etc.

Source: Weka FAQ