Free Data Mining Tools
Weka - an open-source software for data mining
RapidMiner - an open-source system for data and text mining
KNIME - an open-source data integration, processing, analysis, and exploration platform
The Mahout machine learning library - mining large data sets. It supports recommendation mining, clustering, classification and frequent itemset mining.
Rattle - a GUI for data mining using R
CLUTO - a software package for clustering low- and high-dimensional datasets
fastcluster - fast hierarchical clustering routines for R and Python
TraMineR - an R package for mining and visualizing sequence data
Social Network Analysis
Gephi - an interactive visualization and exploration platform for networks and complex systems, dynamic and hierarchical graphs
Pajek - a free tool for large network analysis and and visualization
CFinder - a free software for finding and visualizing overlapping dense groups of nodes in networks, based on the Clique Percolation Method (CPM)
ProM - a free software for process mining
Spatial Data Analysis
The follows are some free and/or open source tools for data mining applications. Some of them are free for non-profit use only. Please check corresponding websites for license details.