Apache Mahout is an open source machine learning library built in Java. It provides algorithms for recommendation, clustering, and classification. Some key benefits of Mahout include its Apache license, active community support, good documentation, and ability to scale to large datasets using Hadoop. It supports many common machine learning algorithms such as collaborative filtering, k-means clustering, logistic regression, and neural networks. While other options like Weka and R exist, Mahout is preferable for its scalability on big data using Hadoop.