Besides Java class reloading JRebel provides the integration for the wide range of technologies in the Java ecosystem. By that I mean integration with all kind of containers and frameworks.
The integration with containers is needed in order to be able to work in the Java EE world. For instance if you want to add a new EJB on the fly. To support framework-specific changes there are special plugins incorporated with JRebel. For instance if you want to add a new bean in Spring context and get it injected somewhere else.. or add new Struts actions on the fly... or see the changes to your Java classes in JSP scriplet. There's a lot of features to list
The full list of the specific framework integrations is available on the website