The people at Alias-i, who provide the Natural Language Processing package LingPipe (mostly used now for biomedicine, providing a suite of NLP tools - Part Of Speech Tagging, Named Entity Recognition, etc.), have kindly collected a list of NLP packages, including a big number of opensource ones.
I love this comment they make:
A good point for LingPipe people is that they provide an extensive number of tutorials on so many appealing topics that you could spend a month there testing and testing. Just do not miss the one on Sentiment Analysis (a black pearl in a box of white pearls!).
Search, Speech, Translation, OCR, ...? We have intentionally not listed competitors focused on things other than basic language processing tools. Companies in these businesses are more likely to be LingPipe customers than LingPipe competitors.