Relax ng tools
From PERI
Tools for dealing with RELAX-NG
You can convert RELAX-NG (.rnc) schemas to XML Schema with Trang [1]. This allows use of the XML-Schema toolset. To check against the XML-Schema schema, you can use something like Oxygen (http://www.oxygenxml.com) on *nix, or XML Spy (http://www.altova.com/simpledownload1.html) on Windows. Neither is free, but I think you can get a trial license.
To check the XML against the RELAX schema, you can use a validator like RNV (http://ftp.davidashen.net/PreTI/RNV/) or if you have a Mac you can try XML Nanny (http://xmlnanny.com/). Another one is Sun's Multi-Schema Validator (MSV) which is free but requires registration, a download link is at: http://sdlc5b.sun.com/ECom/EComActionServlet;jsessionid=4AF8887329D96ED6A13E91BB6E7FEB68
If you're writing out test documents by hand, I use the "nXML" mode [1] in Emacs (and the "rnc" mode to modify the schema).
[1] Both trang and nxml are available from: http://www.thaiopensource.com/download/ . The nxml manual is at http://infohost.nmt.edu/tcc/help/pubs/nxml/index.html
