Also have a look at there Graphical XSD editor (also FREE), we've been
using it for quite a while now, and its pretty much on a par with
XmlSpy, and blows Stylus studio out the water.
Free is good, sometimes.
And if "blows Stylus Studio" out of the water means "it looks a
little prettier," we're in agreement.
But it lacks schema refactoring, schema documentation generation,
and XML instance generation, all of which both spy and Stylus
Studio provide, and it doesn't scale with documents larger than
a few hundred K.
Empirical evidence is good, lists of pros and cons is good,
lists of features supported and not supported is also good.
Subjective statements don't help others make informed decisions
though.
On the original topic, there are lots of good validators.
Xerces from the Apache project is pretty decent, and the Saxonica
validator (
http://www.saxonica.com/) has excellent diagnostics.
The .net validators and XSV also have their place.
Stylus Studio includes all of the above, including Saxon's which
we license. I've found it useful to try validating with more than
one editor, since we don't always know what environment the XSD
will be used in once it is released into the wild. And different
ones have different strengths and often catch different problems.
And most don't suffer from the one-defect-and-I-stop problem.
http://www.stylusstudio.com/xml_schema.html has more information
on validators, including IBM's Schema Quality Checker and Sun's
Multi-Schema Validator.
A free evaluation copy of Stylus Studio is available from
http://www.stylusstudio.com/xml_download.html which includes
all of the above-mentioned validators. Try it if only to decide
which validator you'd prefer, since it bundles them all in one
place.