Apache Mahout(TM) is a distributed linear algebra framework and mathematically expressive Scala DSL designed to let mathematicians, statisticians, and data scientists quickly implement their own algorithms. Apache Spark is the recommended out-of-the-box distributed back-end, or can be extended to other distributed backends.

Mathematically Expressive Scala DSL
Support for Multiple Distributed Backends (including Apache Spark)
Modular Native Solvers for CPU/GPU/CUDA Acceleration