Apache Mahout is described as 'Apache project to produce free implementations of distributed or otherwise scalable machine learning algorithms on the Hadoop platform'. There are four alternatives to Apache Mahout for Linux, Mac, Windows, BSD and Self-Hosted solutions. The best alternative is WEKA, which is both free and Open Source. Other great apps like Apache Mahout are ELKI (Free, Open Source), KEEL (Free) and SIMON (Free, Open Source).
ELKI: "Environment for Developing KDD-Applications Supported by Index-Structures" is a development framework for data mining algorithms written in Java. It includes a large variety of popular data mining algorithms, distance functions and index structures.