Update This description has been updated in the following tutorial: Eclipse e4 tutorial.
In the Eclipse e4 webseminar I learned about the e4 UI editor which allow you to change the user interface on the fly.
As you most likely know the Eclipse UI is modelled as an Eclipse EMF model. This model can be changed during directly in Eclipse via the editor or via the EMF API (I have not yet tested this) and the UI will change immeadiately according to the change.
To following my example you need the E4 0.9 download from Eclipse E4 0.9 download.
Start Eclipse e4 tutorial. Select Window -> UI Editor
Open the WorkbenchWindow
You can now modify the model, e.g. you can drag the package explorer on the same level as the Java perspective.
I believe the modeled UI concept is really powerful as it exposes the internal state of the Eclipse based apllication via a well-defined model with a clear API.
Kudos to the Eclipse e4 team!