Natural Language Processing libraries

https://opennlp.apache.org/ http://nlp.stanford.edu/

Cloud provider automation libraries

http://jclouds.incubator.apache.org/

Distributed stream computing

https://incubator.apache.org/s4/

Asynchronous event driven IO, battle tested, mother recommended

http://netty.io/

In-process webserver

http://www.eclipse.org/jetty/

....to name a few. It's probably useful to take some thing that you'll need and look in ecosystem X for it (say... .NET) and then compare what's available there to what's available on the JVM. In my experience, the JVM wins out almost every time when it comes to number of options and maturity of options.