Eclipse has tools for doing this (and it's free.)
EDIT (yeah, I was a little too terse) : What you want are the XSD editing tools in Eclipse. I know it's bundled with Eclipse IDE for Java EE Developers, and I think also with the Eclipse Modeling Tools download. (It's also possible to add them into an existing Eclipse install, though I don't know exactly which plugin(s) you'll want to add.)
(I'd like to be more precise than that, but the eclipse.org web site models itself after Massachusetts roads: If you don't know where you are, you don't belong there.)
Anyway: Once you've got the right version of Eclipse, open the existing schema file for editing (or create a new one: select File -> New... Other ... XML / XML Schema ). When you're ready to generate a test XML file, locate the file in the Package Explorer (the navigator view, usually on the left side), right click on it, and select Generate/XML File.
(What was I saying about navigability... ?)