Some libraries are:
Scikit-learn: comprehensive and easy to use, I wrote a whole
article on why I like this library.
PyBrain: Neural networks are one thing that are missing from
SciKit-learn, but this module makes up for it.
nltk: really useful if you’re doing anything NLP or text
Theano: efficient computation of mathematical expressions
using GPU. Excellent for deep learning.
Pylearn2: machine learning toolbox built on top of Theano -
in very early stages of development.
MDP (Modular toolkit for Data Processing): a framework that
is useful when setting up workflows.