The easiest way to install a plugin is going to "Tools" -> "Plugins" -> "Available" and using the search and the right upper corner of the dialog:
1. UML Plugin. With this feature existing code can be visualized using UML. The diagramms can be even updated - in case the code changes afterwards.
Netbeans will create a new context menu "reverse engineer" for you. "Reverse Engineer" creates a UML-projects and imports all classes from the source folder. This procedure can take some seconds. Having imported the classes, go to the UML-project select the interesting ones and choose "Create diagram from selected elements". The Feature "Generate Dependency Diagram" is interesting as well.
2. Java FX Script support with the weather sample: Install the JavaFX Plugins, as well as the whether sample plugin. Afterwards restart the ide an choose the JavaFXWeather from the samples node. The look and feel and behavior of the application is really amazing. It looks like Vista's widget bar. Even better is the Java2D tutorial. Just start the tutorial project. It comes with many Java FX samples.
3. Support for RestFul services. This plugin installs a project which demonstrates the use of JSR-311 (Restful Services). A funny story here: in an Java EE 5 workshop I explained the ideas of RESTful services. During this time a student was able to install the plugin as well as deploy it to glassfish v2. He showed me the result after my explanation :)
4. JMeter Kit: the integration of Apache's JMeter into netbeans.
5. JMX integration. It starts the JConsole from the IDE and saves so some roundtrips outside netbeans.
6. Eclipse/JBuilder Importer: they are working well, however I prefer to import eclipse's projects by hand.