From NLTK

NLTK includes the following software modules (~120k lines of Python code):

Corpus readers

interfaces to many Corpora

Tokenizers

whitespace, newline, blankline, word, wordpunct, treebank, sexpr, regexp, Punkt sentence segmenter

Stemmers

Porter, Lancaster, regexp

Taggers

regexp, n-gram, backoff, Brill, HMM

Parsers

recursive descent, shift-reduce, chunk, chart, feature-based, probabilistic, ...

Semantic interpretation

untyped lambda calculus, first-order models, parser interface

Wordnet

wordnet interface, lexical relations, similarity, interactive browser

Classifiers

decision tree, maximum entropy, naive Bayes, Weka interface, megam

Clusterers

expectation maximization, agglomerative, k-means

Evaluation

accuracy, precision, recall, windowdiff

Estimation

uniform, maximum likelihood, Lidstone, Laplace, expected likelihood, heldout, cross-validation, Good-Turing, Witten-Bell

Miscellaneous

feature detection, unification, chatbots, many utilities

NLTK-Contrib (less mature)

categorial grammar (Lambek, CCG), dependency parser, finite-state automata, glue semantics, hole semantics, hadoop (MapReduce), kimmo, readability, textual entailment, timex, TnT

Browse the source code: http://nltk.org/nltk/

Browse the subversion repository: http://code.google.com/p/nltk/source/browse/trunk/nltk