Jason Mirra has a nice article on Fast and Effective GUI Building in Java using NetBeans.

And, Igor pointed to some Visio stencils at http://c2.com/cgi/wiki?GuiPrototypingTools.

For future reference:

• http://www.balsamiq.com/products/mockups#
• http://www.evolus.vn/Pencil/
• http://www.mockupscreens.com/
• http://visualwx.altervista.org/
• http://wxglade.sourceforge.net/
• http://www.projectspace.nl/
• http://tof2k.com/ext/formbuilder/
• http://dub.washington.edu/denim/
• http://www.serena.com/products/prototype-composer/home.html
• http://www.omnigroup.com/applications/OmniGraffle/
• http://napkinlaf.sourceforge.net/
• Microsoft Visio, PowerPoint, Visual Studio WinForms,
• http://www.istartedsomething.com/20071018/powerpoint-prototype-toolkit-01/
• http://www.krisjordan.com/2008/09/07/10-minute-mock-prototyping-tips-for-powerpoint/
• http://www.youtube.com/watch?v=JjbeCkn0bJg
• Article by Eric Burke at http://stuffthathappens.com/blog/2008/07/26/balsamiq-mockups/
• http://developer.yahoo.com/ypatterns/wireframes/
• http://nickfinck.com/stencils/
• http://www.adobe.com/products/fireworks/
• http://www.adobe.com/devnet/fireworks/articles/gallery_mockup_print.html
• http://glade.gnome.org/ 
• Qt Designer
• NetBeans Gui Builder
• Py4Qt
• http://stackoverflow.com/questions/tagged?tagnames=gui