ScalNet is the Keras-like Scala API for Deeplearning4j.
Skymind Intelligence Layer CE is a free machine learning platform that offers Scala notebooks with Zeppelin, which rely on Apache Spark for distributed training.
SMILE, Haifeng Li’s Statistical Machine Intelligence and Learning Engine, includes a Scala API and relies on ND4J/ND4S for numerical computation.
Cortex - Twitter uses Scala for much of its big data stack.
MLlib is part of Apache Spark. While it includes several popular machine learning algorithms, it is not archtected to perform computation efficiently. For better architectures using Spark for machine learning, here is Deeplearning4j’s integration with Apache Spark for distributed neural net training.
MxNet has a Scala API.
ND4S, or n-dimensional arrays for Scala, is a tensor library written in Scala that performs scientific computing functions much like Numpy or Tensorflow.
Breeze is a numerical computing library written in Scala. Unlike ND4J/ND4S, it does not support n-dimensional arrays, or tensors.