Xlink describes a standard way to add hyperlinks to an XML
file.
XPointer and XFragments are syntaxes in development
for pointing to parts of an XML document. An XPointer is a bit like
a URL, but instead of pointing to documents on the Web, it points
to pieces of data inside an XML file.
XSL is the advanced language for expressing style sheets.
It is based on XSLT, a transformation language used for rearranging,
adding and deleting tags and attributes.
DOM is a standard set of function calls for manipulating
XML (and HTML) files from a programming language.
XML Schemas 1 and 2 help developers to precisely
define the structures of their own XML-based formats. There are
several more modules and tools available or under development. Keep
an eye on W3C's technical reports page.