One of the new features in JDeveloper 11g is a simpler extension development process.
This is actually a collection of dialogs that makes creating the artifacts that make a JDeveloper extension an automatic process.
Brian Duff did a video of this back in the technology preview days - and it seems the video is down now. So, I created a new 5 minutes online demo that shows you how simple it is to create an extension to JDeveloper.
You can learn more about extension development for JDeveloper here.