Free NLP Tools for SEO

Amalgram – it is a pos Tagger
Assert  – it’s for semantic role tagging.  It annotates naturally occurring text with semantic arguments.
FreeLing –  it’s a package for language analysis containing amongst other things sentence splitters, pos-taggers, morphological analysis, flexible multiword recognition, named entity detection.
GATE  – for general text engineering, an awesome toolkit for text-mining.
LingPipe – Java libraries for linguistic analysis.  It uncovers relationships within your text, and classifies text passages by language, character encoding, genre, topic, or sentiment and it can also cluster documents into sets.
Naive-Bayes algorithm from CMU – good for text classification, they’re a little more AI because they learn.
SenseClusters – clusters similar contexts together (using unsupervised methods)
Statistical Language Modelling Kit – for playing with n-grams, discounting schemes and so on.
WordSmith Tools – lots of language tools in one environment, the text appears all highlighted and analyzed to use.
WordNet  – little machine readable dictionary, but not good for domain specific tasks.