We did not cover every machine learning package available for Python. Given the limited space, we chose to focus on scikit-learn. However, there are other options and we list a few of them here:
MDP toolkit (http://mdp-toolkit.sourceforge.net): Modular toolkit for data processing
PyBrain (http://pybrain.org): Python-based Reinforcement Learning, Artificial Intelligence, and Neural Network Library
Machine Learning Toolkit (Milk) (http://luispedro.org/software/milk): This package was developed by one of the authors of this book and covers some algorithms and techniques that are not included in scikit-learn
Pattern (http://www.clips.ua.ac.be/pattern): A package that combines web mining, natural language processing, and machine learning, having wrapper APIs for Google, Twitter, and Wikipedia.
A more general resource is http://mloss.org, which is a repository of open source machine learning software. As is usually the case with repositories such as this one, the quality varies between excellent well maintained software and projects that were one-offs and then abandoned. It may be worth checking out whether your problem is very specific and none of the more general packages address it.