If you need to do work with schema stuff (RelaxNG, Schematron, or XML Schema. Oh, and DTDs but you don't want those of course) and you're running windows, I highly recommend trying out the Topologi Validator.
Currently, the only thing worse than the XML Schema specification is the XML Schema toolset. Almost all of the tools are terribly limited, buggy, incomprehensible, etc. (check all that apply) and while Topologi isn't really an exception in that it uses buggy MSXML underneath, it does manage to make a lot of that bearable.
Thankfully, Andy Wardley told me that his XML::Schema module should be out soon. I'm pretty certain that it'll make working with schemata a lot easier.