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, and an active discussion forum.

Thanks to a hands-on guide introducing programming fundamentals alongside
topics in computational linguistics, NLTK is suitable for linguists,
engineers, students, educators, researchers, and industry users alike.
NLTK is available for Windows, Mac OS X, and Linux.  Best of all, NLTK is
a free, open source, community-driven project.

NLTK has been called "a wonderful tool for teaching, and working in,
computational linguistics using Python" and "an amazing library to play
with natural language".

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