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There's waffles http://waffles.sourceforge.net/

and vowpal wabbit https://github.com/JohnLangford/vowpal_wabbit/wiki

I think the juggernaut here is WEKA [0], I've seen it used in the real world far more than anything else.

[0] - http://www.cs.waikato.ac.nz/ml/weka/

LensKit: http://lenskit.grouplens.org which also prominently features in the currently running 'Intro to Recommender Systems' MOOC on Coursera.

The MOOC course is very interesting. Do you know if there's a way to get enrolled? It looks like the registration is closed and there's no upcoming offerings available currently. Thanks.