NLTK (Natural Language Toolkit)
"NLTK is a leading platform for building Python programs to work with human language data. It provides easy-to-use interfaces to over 50 corpora and lexical resources such as WordNet, along with a suite of text processing libraries for classification, tokenization, stemming, tagging, parsing, and semantic reasoning, wrappers for industrial-strength NLP libraries, and an active discussion forum."
— NLTK website
NLTK is a free library that helps a wide variety of tasks in NLP, from tokenizing, to integrating large corpora, to displaying parsing trees and much more.
Code Examples
The NLTK website has numerous detailed demonstrations of all its functions.