ACOPOST is a set of freely available POS taggers modeled after well-known techniques. The programs are written in C (aiming for extreme portability and code correctness/safety) and run under various Unix flavors (and probably even under Windows). ACOPOST currently consists of four taggers that are based on different frameworks: Maximum Entropy Tagger (MET), Trigram Tagger (T3, based on Hidden Markov Models), Error-driven Transformation-based Tagger (TBT or Brill Tagger), and Example-based tagger (ET).
Apache OpenNLP is a machine learning based toolkit for the processing of natural language text. It supports the most common NLP tasks, such as tokenization, sentence segmentation, part-of-speech tagging, named entity extraction, chunking, parsing, and coreference resolution. These tasks are usually required to build more advanced text processing services.