WindowBuilder (Swing, SWT, GWT etc.) was a commercial product before Instantiatons was acquired by Google and has been proposed as a new project to the Eclipse Foundation. While you'll certainly always have the greatest flexibility with hand coding (like Aaron suggests in his answer) I have had a lot of great experience with the tool.
The project is in Eclipse Incubation currently, but has been used in production for years. If you want to create a GUI with Drag'n'Drop you should definitely have a look at WindowBuilder.