NLTK

The Natural Language Toolkit (NLTK) provides access to corpora and lexical resources, and to a complete suite of functions for Natural Language Processing (NLP), ranging from tokenizers to part-of-speech taggers and from tree models to named-entity recognition. Initially, Steven Bird and Edward Loper created the package as an NLP teaching infrastructure for their course at the University of Pennsylvania. Now it is a fantastic tool that you can use to prototype and build NLP systems:

• Website: http://www.nltk.org/
• Version at the time of print: 3.3
• Suggested install command: pip install nltk