MyEclipse IDE comes with a nice GUI builder Matisse4MyEclipse ("borrowed" from NetBeans IIRC). You will probably want to settle on a Java IDE as part of the decision making process since the two decisions tend to be closely related.
Instantiations is also another solid choice I have used, though a bit more expensive. CodeGear apparently likes this tool as well since it has been selected for inclusion in their JBuilder product lineup (which is now based on Eclipse).
Another option you should not overlook is Eclipse Rich Client Platform (RCP). Depending on who you ask it is either wonderful or evil since it competes with Swing and makes use of Native OS widgets (SWT). This is the technology behind Eclipse itself and may be a good choice if you like the Eclipse look and feel or are building a tooling platform.
FWIW I have used MyEclipse tooling extensively since about 2004 and am happy with the quality and very reasonable price. Please keep in mind that YMMV since everyone has different preferences and requirements.
