Netbeans (www.netbeans.org) is free and Open Source. I think you will
find it the best choice. Borland JBuilder Personal Edition
(http://www.borland.com/jbuilder/personal/index.html) is also free (or
$10.00 if you want it on CD). Oracle's JDeveloper
(http://otn.oracle.com/products/jdev/content.html) is free but the
license severely restricts what you can do with programs you write if
you do not pay for it.

Eclipse (www.eclipse.org) is a popular Open Source IDE and there are
free plug-ins like http://www.assisiplugins.com/index_start.html and
http://www.eclipse.org/vep/ that provide limited WYSIWYG GUI
development.

Sun One Studio 4 Community Edition, which is derived from Netbeans, is
still available, without charge, from
http://jsecom16d.sun.com/ECom/EComActionServlet?StoreId=8&PartDetailId=FJCI9-401
-TL9M&TransactionId=try&LMLoadBalanced=
but has been superseded by Studio 5. Sun suggest you use Netbeans if
you want a free version. IBM has a free version of Visual Age for Java
at
http://www7.software.ibm.com/vad.nsf/FrameData/Master?OpenDocument&FSet=1&Doc3=4
589&Title=Overview&DPart=Overview&Doc4=4594.
It has a limit on the number of classes in your application and, in
any case, has been superseded by Websphere Studio Application
Developer (http://www-3.ibm.com/software/awdtools/studioappdev/).
Websphere is not free but a sixty day trial is available.

I believe that this is a complete list of free Java IDE's that include
a WYSIWYG GUI builder and contains references to all the commercial
vendors of such a product. Please submit a follow-up if you have a
correction.