According to the yearly KDnuggets Polls 2007, 2008, and 2009, RapidMiner is the most widely used Open Source Data Mining Solution among data mining experts world-wide:
KDnuggets Data Mining Tool Poll 2009
RapidMiner is open source and 100% Java, RapidMiner is much more flexible and offers significantly more functionality than Weka and KNIME.
