I don't know what do you mean by "end-user facing specs/apps".
But if enterprise apps where the content is formatted in XML count, then there are thousands (I'm working on OpenERP, for example, which uses XML both to define views and to transmit content to the client).
Then there's SVG. And the Office formats, both MS' and LibreOffice's. And obviously XHTML. And XMPP, which drives Google Talk and clients to FB Chat.
But if enterprise apps where the content is formatted in XML count, then there are thousands (I'm working on OpenERP, for example, which uses XML both to define views and to transmit content to the client).
Then there's SVG. And the Office formats, both MS' and LibreOffice's. And obviously XHTML. And XMPP, which drives Google Talk and clients to FB Chat.
Just look at the list: https://en.wikipedia.org/wiki/List_of_XML_markup_languages