Mahout currently has
Collaborative Filtering
User and Item based recommenders
K-Means, Fuzzy K-Means clustering
Mean Shift clustering
Dirichlet process clustering
Latent Dirichlet Allocation
Singular value decomposition
Parallel Frequent Pattern mining
Complementary Naive Bayes classifier
Random forest decision tree based classifier
High performance java collections (previously colt collections)
A vibrant community
and many more cool stuff to come by this summer thanks to Google summer of code
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S. Pohl, F. Radlinski, and T. Joachims. Proceedings of the 7th ACM/IEEE-CS Joint Conference on Digital Libraries, page 417--418. New York, NY, USA, ACM, (2007)