Liquid XML Studio is a FREE XML Schema Editor, and much more.
Simple enough for new users to pick up quickly and comprehensive enough for experienced users deal with the advanced features of the XSD standard.
KG: Free, but not open source. For the quick schema edit...
Cogenit's Xcarecrows 4 XML for Eclipse completes today's powerful integrated development environment to handle the daily tasks required by an XML workflow. To this end, Xcarecrows 4 XML offers:
* a graphical XML, XML Schema and XML stylesheets editor ;
* a graphical XML tree comparator ;
* a built-in checker against XML Schemas ;
* an XSL transformations tool kit.
Pedro is an application that creates data entry forms based on a data model written in a particular style of XML Schema. Users can enter data through the forms to create data files that conform to the schema. They can use controlled vocabularies to mark-up text fields and have the application perform basic validation on field data. When they feel they have finished writing a data file, Pedro can tell them if they have left out any required records.