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://nltk.sourceforge.net/