Date: May. 15, 1999
Link: http://msdn.microsoft.com/xml/notepad/intro.asp
Source Author or Organization: Microsoft

There is a free beta available for download of Microsoft® XML Notepad, a simple prototyping application for HTML authors and developers designed to enable the rapid building and editing of small sets of XML-based data.

XML Notepad let's you create elements and attributes and fill in their values, but it does not let you actually edit the source. It generates well-formed source when an .xml file is saved.