I am looking for an open source Natural Language Processing library for c/c++ and especially i am interested in Part of speech tagging.
Take a look at this POS Tagger list from Stanford. Some of them are language independent and others are targeted at C/C++ or have specific bindings.
Not present on that list, but still important in my opinion is Citar, a C++ free software part of speech tagger using a trigram Hidden Markov Model.