Command line query tools
GUI query tools
JSP, ASP, PHP, and other such server side technologies
Programs written in Java, C++, and other languages that need to extract data from XML documents
Others are possible
Anywhere SQL is used to extract data from a database, XQuery is used to extract data from an XML document.
SQL is a non-compiled language that must be processed by some other tool to extract data from a database. So is XQuery.