Apache Mahout is an open source machine learning library that provides scalable machine learning algorithms focused on clustering, classification, and collaborative filtering. It allows building scalable machine learning tools for analyzing big data in a distributed manner using frameworks like Hadoop. Some key algorithms supported include logistic regression, Bayesian classification, k-means clustering, and item-based collaborative filtering. Companies are using Mahout for applications like movie recommendations, fraud detection, and ad recommendations by taking advantage of its scalability for large datasets.