Rhino

Rhino is a JavaScript interpreter written in Java. Started by Netscape as a component of their since long-deceased plan for implementing a browser in Java, Rhino survived the axing of that project and took on a successful independent life of its own. I chose Rhino in 2004 as the scripting language for a big distributed workflow system I was designing at the time at work. The system is since in heavy enterprise-grade production use, and we're very happy with Rhino. As we were using it, we found bugs and I spotted several opportunities for optimizations or furthering scalability (a valid concern in server side scripting), which led me to contributing patches to it, to the net effect that I became a committer on the project in 2005 and subsequently took over as its maintainer in 2006.