Open source Python modules, linguistic data and documentation for research
and development in natural language processing, supporting dozens of NLP
tasks, with distributions for Windows, Mac OSX and Linux.

NLTK comes with many corpora, toy grammars, trained models, etc. A complete
list is posted at: http://nltk.googlecode.com/svn/trunk/nltk_data/index.xml
To retrieve all the data, use "python -m nltk.downloader all".