Alternative schemas

Schema-free A lot of projects don't use any form of formal constraint mechanism.
DTDs DTDs will likely be popular for a few years to come.
XDR, SOX, and friends These predecessors to W3C XML Schema are already in use, and some developers have said "if it ain't broke, don't fix it."
RELAX A challenge to W3C XML Schema from Japan, moving through ISO. Has a very different vision of typing, and doesn't modify document content.
TREX A challenge to W3C XML Schema from James Clark (creator of XSLT) and OASIS. Similar foundations to RELAX, with more functionality for some cases.
Schematron Built by Rick Jelliffe on XSLT, Schematron provides reports about the content of a document. Allows you to set your own error messages.
Examplotron Built by Eric van der Vlist, allows annotated example documents to serve as schemas. (Blindingly obvious, but surprisingly powerful.)