BorgeIt Data Mining Suite by University of Magdeburg

Techniques: apriori, bayesian network, decision trees, regression, clustering

Good for general purpose data mining

R by R Foundation

Techniques: linear and nonlinear modeling, classical statistical tests, time-series analysis, classification, clustering, boosting, random forests

Seems to be for statistical research

Rattle by Togaware

Techniques: apriori, decision tree, generalized linear models, oosting, random forests

General data mining application; looks simple to use

WEKA--The Waikato Environment for Knowledge Analysis

Techniques: Lots. decision trees, association rules, clustering, random forests, support vector machines

Java. Pretty basic interface. Looks powerful

C4.5 Tutorial, Original

Techniques: C4.5 is a decision tree generating algorithm

It appears you have to compile source code in order to run the tool. Most off-the-shelf data mining tools come with a freely available implementation of the decision tree algorithm, such as WEKA

AlphaMiner by E-Business Technology Institute