Trang - Multi-format schema converter based on RELAX NG. Trang can also infer a schema from one or more example XML documents.
jing-trang - Schema validation and conversion based on RELAX NG - Google Project Hosting
RELAX NG home page - RELAX NG is a schema language for XML.
Jing - A RELAX NG validator in Java
DTDinst - DTDinst is a program for converting XML DTDs into XML instance format.
XMLBeans Tools - XMLBeans includes several command-line tools you might find handy as shortcuts for common tasks.
Welcome to xmltwig.org - XML::Twig: XML, The Perl Way. subclasses XML-Parser to allow easy processing of XML documents of all sizes.
Comparison of XML editors - Wikipedia, the free encyclopedia
Commercial XML editorsAltova XML Editor and Award-winning XML, SQL, and UML Tools
oxygenxml XML Editor - $$$
XML Editor, XML Tools, and XQuery - Stylus Studio