One recent cool thing about designing your own windows (either in order to extend the IDE or as part of your own application on top of the NetBeans Platform) is that you can use the GUI Builder to do so. And, once you realize that you can (a) create an NBM file whenever you want and (b) install that NBM file into the current IDE whenever you want, you've got a very powerful testing facility. For example, below I'm adding GUI components to the default (i.e., blank) window that the Window Component wizard creates for me. You can see that I've installed my half-finished product -- since I specified that I want the window to be docked in the Navigator, that's where it is. So, meanwhile, I continue extending my window component in the design view of the GUI Builder (using Matisse, also known as the 'Free Design' layout) and whenever I want to see the result I create an NBM file, install it via the Update Center, and immediately see the result (click to enlarge):
Of course, use the above technique at your own risk -- bear in mind that if your module is really screwy, you could completely destroy the current instance of the IDE, including the work you're currently engaged in. Therefore, it's probably better to choose 'Install/Reload in Target Platform' (instead of 'Create NBM'). But, then again, that would mean that a whole new instance of the IDE has to start up. Plus, it's just plain cool to see my half-finished product in action in the IDE while I'm simultaneously still working on it within the same IDE instance.