What are some alternatives to Mahout?

Hello Sean, Robin, Ted, and Ellen,

I am wondering: what are some alternatives to Mahout that are mature, full-featured, and well-developed? It doesn't have to be strictly Java-based. I am interested to know of similar projects for any programming language and toolset.

The reason I ask is that I have often found it helpful to examine related projects' documentation, examples, source code, and mailing lists to gain a better understanding of a particular project.

It depends on what you are doing.

For smaller-scale problems, Ted rightly always recommends R for playing around.
Weka is a quite established and mature library for machine learning.

For collaborative filtering I know of Cofi, CoFE, Duine, Vogoo, and more as potentially useful libraries -- depends on what language, environment and use case you have.

Owen is correct. I always do recommend R.

In fact, I also use R quite a lot. It is really valuable as a reference implementation or for exploratory use. For production, I very much prefer Mahout.

Another interesting related project is Vowpal Wabbit. Their focus is even more narrow than Mahout's and it is much harder to integrate VW models into a working system. See http://hunch.net/~vw/ for more information.