I second eserte's recommendation to use trang. (NM, just read the rest of the thread... :-()

I think Altova XML (XML Spy) also does what you're looking for. A tour of the W3C XML Schema section may benefit you. You may also be interested in the XML Schema schema for XML Schemas (there may well be a newer version; this just happens to be my bookmark from way back when).

Update: Maybe try the DTDGenerator - A tool to generate XML DTDs