Mlpack: A C++ machine learning library (github.com)

I've used it, it's great. My main interest was using some of the regularization methods, which all worked as expected.

I was able to talk to some of the maintainers on IRC, very responsive and tolerant of my stupid questions.

Here's a list of some (academic) uses of mlpack:

http://scholar.google.ca/scholar?cites=17794023163287158191&...

Development of mlpack has been going on for at least 7 years and my impression is that it's pretty mature. It was originally affiliated with a lab at Georgia Tech, where its current maintainer (a friend of mine) is a PhD student. The dual-tree methods are based on his research.