REST Java Client for JIRA
This is not a plugin per se. It's a Java client library (usuable from any JVM-based language like Scala, Groovy, JRuby, etc.).
The purpose of this library is to make JIRA REST API very easy to use on the client side.
JRJC removes the need of traversing URIs, preparing requests and parsing the output.
This project is not actively supported by Atlassian, but feel free to contribute to it.