You can find the list of extensions available for your version of JDeveloper through the help->Check for Update menu option.
You can also check out these update centers:
You can find a list of extensions for previous versions here.
Oracle provides an extension development kit with samples and documentation to help you build your own extensions.
We invite you to contribute to this exchange by sharing your own JDeveloper extensions. Please post an entry on the discussion forum if you'd like to upload your extension on this page.