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.
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.