OpenNLP has a nice and easy to understand API. The library is usable as a stand alone command line tool but can also be used within products which makes it easy to test new ideas on the CLI and the transition to productionizing a prototype is quick. Review collected by and hosted on G2.com.

The project has been abandoned and is no longer up to date. There's also no indication in any parts of the API as to which functions and classes are thread safe versus not. I wish there was more example code in the documentation, and the models that have been trained that are available for download are from very long ago. Review collected by and hosted on G2.com.