XML Schema Validating Parser

This may be a dumb question but worth a shot anyway.  Does anyone know
who the frontrunner is for releasing a schema validating parser?  That
is, a parser that validates xml based on  a schema (checks datatypes,
occurances, etc.).  I realize that until the specification is finalized
that I won't see a universal parser of this kind any time soon.  I
would, however, like to know which groups out there are close to
finishing one or perhaps sitting on one already.  Any info would be
greatly appreciated.