DTDs and validity are most important when you're exchanging data with others; they let you verify that you're sending what the receiver expects and vice versa. Of course, this works best if both ends of a conversation agree on which DTD and vocabulary they will use. There are many standard DTDs for different professions and disciplines and more are created every day. It is often better to use an established DTD and vocabulary than to design your own. However, there is no agreed-upon, central repository that documents and links to such efforts. The largest list of DTDs online is probably Robin Cover's list of XML applications at http://www.oasis- open .org/cover/siteIndex.html#toc-applications.
The W3C is one of the most prolific producers of standard XML DTDs. It has moved almost all of its future development to XML, including SVG, the Platform for Internet Content Selection (PICS), the Resource Description Framework (RDF), the Mathematical Markup Language (MathML), and even HTML itself. DTDs for these XML applications are generally published as appendixes to the specifications for the applications. The specifications are all found at http://www.w3.org/TR/.
However, XML isn't just for the Web, and far more activity is going on outside the W3C than inside it. Generally, within any one field, you should look to that field's standards bodies for DTDs relating to that area of interest. For example, the American Institute of Certified Public Accountants has published a DTD for the Extensible Financial Reporting Markup Language (XFRML). The Object Management Group (OMG) has published a DTD for describing Unified Modeling Language (UML) diagrams in XML. The Society of Automotive Engineers has published an XML application for emissions information as required by the 1990 U.S. Clean Air Act. Chances are that in any industry that makes heavy use of information technology, some group or groups, either formal or informal, are already working on DTDs that cover parts of that industry.