Oracle JDeveloper's Extension SDK is a public API that can be used to extend and customize the JDeveloper development environment. Developers can use the Extension SDK to develop extensions and integrate third party tools with the JDeveloper IDE. Using the APIs provided by the Extension SDK developers can add components that range from a new menu option through a wizard and up to a complete new editor. Extensions can be exposed through JDeveloper's Check-for-Updates functionality for easy one click install.
With the release of JDeveloper 11gR2, there is a difference between the two versions of the Extension SDK.
ESDK v2.0 is for 11.1.2.x versions of JDeveloper (11gR2)
ESDK v1.0 is for 11.1.1.x versions of JDeveloper (11gR1)
For the most part, the examples and getting started reference are the same. It will be noted where there is a difference.
JDeveloper offers a set of sample extensions complete with their code as well as comprehensive online documentation for building extensions. These are available for download through the help->check for updates menu option in JDeveloper. Further support is provided through the JDeveloper discussion forum. A quick demo of building extensions for JDeveloper 11gR2 is here, and for 11gR1 here. For 11gR2 you can find a new Developers Guide for Oracle JDeveloper Extensions.