Java GUI Builder
One of the things I really like about PythonCard is that it enables (and in fact actively encourages) you to completely separate the GUi of your application from the program logic. In PythonCard you design your GUI by adding and dragging elements around in the resource editor, then create a simple Python class with event handlers to define what should happen when your GUI is interacted with. Now, thanks to the Java GUI Builder (spotted on Small Values of Cool), you can do the same thing in Java.
The GUI builder allows you to define your user interface in an XML file, which is then interpreted by Java at run-time. This allows you to make changes to the interface without recompiling the program (or editing any Java code), and also gives users the ability to edit their interface without needing to alter your code. It’s a very clever piece of code.
XML seems to be a popular choice for defining GUI’s these days—Mozilla has XUL, XWT allows developers to write web based GUI applications with XML and now the Java GUI Builder takes this trend a step further. It seems like a logical progression—after all, GUIs are basically collections of nested components and XML fits that kind of situation like a glove.