NLTK, the Natural Language Toolkit, is a suite of Python libraries and
programs for symbolic and statistical natural language processing.
NLTK includes graphical demonstrations and sample data. It is accompanied
by extensive documentation, including tutorials that explain the underlying
concepts behind the language processing tasks supported by the toolkit.

NLTK is ideally suited to students who are learning NLP (natural language
processing) or conducting research in NLP or closely related areas,
including empirical linguistics, cognitive science, artificial intelligence,
information retrieval, and machine learning. NLTK has been used successfully
as a teaching tool, as an individual study tool, and as a platform for
prototyping and building research systems.

WWW: http://www.nltk.org/