spaCy is a free open-source library for Natural Language Processing in Python. It features NER, POS tagging, dependency parsing, word vectors and more.
NLTK is a leading platform for building Python programs to work with human language data. It provides easy-to-use interfaces to over 50 corpora and lexical resources such as WordNet, along with a suite of text processing libraries for classification, tokenization, stemming, tagging, parsing, and semantic reasoning.
R. Rehurek, and P. Sojka. Proceedings of the LREC 2010 Workshop on New Challenges for NLP Frameworks, page 45--50. Valletta, Malta, ELRA, (May 22, 2010)