TEI (Text Encoding Initiative) is a large, elaborate DTD used primarily in academia for computer transcription of literary texts. TEI's Unix-based toolchains use many of the same tools that are involved with DocBook, but with different stylesheets and (of course) a different DTD.

XHTML, the latest version of HTML, is also an XML application described by a DTD, which explains the family resemblance between XHTML and DocBook tags. The XHTML toolchain consists of Web browsers that can format HTML as flat ASCII, together with any of a number of ad-hoc HTML-to-print utilities.

Many other XML DTDs are maintained to help people exchange structured information in fields as diverse as bioinformatics and banking. You can look at a list of repositories to get some idea of the variety available.