Real Networks and others, to describe a time-based presentation composed of multiple elements - such as adding subtitles to a video. The specific XML type is SMIL, Synchronized Multimedia Integration Language, http://www.w3.org/TR/REC-smil/
Oracle 8i (relational database) has extensive XML support.
http://www.multimania.fr. Most of the 'editorial' content there (articles and the like) is stored as XML. I view it as one great 'success story' for XML (and not just because I wrote their content management system). The main point is that XML allows you, by the judicious use of companion standard XSLT, to separate form and content. Authors don't use HTML; they write articles and other documents as structured, marked-up text. All display attributes such as bold, italics, fonts, image positioning, 'horizontal rule' type separators between sections, whatever... are specified in XSLT stylesheets that are under the sole authority of the graphic designers. Thus, changing content does not involve graphic designers or HTML integrators; conversely, changing style characteristics doesn't involve content authors. This separation allows for a much better publishing process. Also, the process doesn't actually involve DTDs - in practice they turned out not to be necessary.
...but not SGML, which is the heritage of XML.
I can't imagine trying to do a web application these days without XML to transport my data about.
Yep; this is a pretty dated page. ;-> I can recall, when XML was "the amazing new thing!" I kept saying that soon saying "I'll send the data to you in XML format" will be accepted as being just about as normal and meaningless as saying "I'll send the data to you on a nine track tape." Yep; it's happened. (...except that nine track tapes are really, really dead now. ;-)
See: ExtensibleMarkupLanguageCategoryXml, CategoryWhoIsUsing