What are good alternatives to NLTK?
Wikipedia contains an extensive listing of Natural language processing toolkits, under Outline of natural language processing.
See also my recent answer to:
In light of the updated comment, I have presented a metric, see references below:
- NLTK (Natural Language Toolkit) .. Python (240)
 - OpenNLP .. Java (38)
 - MALLET (MAchine Learning for LanguagE Toolkit) .. Java (34)
 - Stanford NLP .. Java (31)
 - LingPipe .. Java (5)
 - FreeLing .. Ruby (4)
 - TreeTagger .. Ruby (2)
 
Note, ClearNLP may be added to the above list:
- ClearNLP .. Java (6)