From the website:
Weka 3: Data Mining Software in Java
Weka is a collection of machine learning algorithms for data mining tasks. The algorithms can either be applied directly to a dataset or called from your own Java code. Weka contains tools for data pre-processing, classification, regression, clustering, association rules, and visualization. It is also well-suited for developing new machine learning schemes.
I would say it is under active development/use since the mailing list archives have an average of about 315 posts per month.
Yes, approximately 315 post per month.
Another tool for your topic map toolbox!