Up to now I have been doing all my Notes 8.5 sidebar UI development manually and coding the UI by hand which is and has been fine. Now getting a WYSIWYG editor is becoming increasingly important both for quick development, demos and for education situations. I went searching for one and found SWT Designer from Instantiations. The editor looks very, very nice (take a look at the demos) and being priced at 239 USD also very competitive from a price standpoint.
My search continues for alternatives but this looks very attractive.