Ben Coffman wrote:

Besides making two completely different systems speak with each other...What would be some applications of XML?

The folks who make the "Analyzer" protocol analyzer have an XML schema for defining network protocols:

http://analyzer.polito.it/30alpha/docs/dissectors/PDMLSpec.htm

There are also XML encoding rules for ASN.1:

http://asn1.elibel.tm.fr/xml/

(Many network protocols, such as SNMP, LDAP, Kerberos, H.323, etc. use ASN.1 to encode the bits that go onto the wire.)

The Mozilla team developed an XML schema (XUL) to describe user interfaces:

http://www.xulplanet.com

You can change Mozilla/Firefox/Thunderbird's interface by dinking with the XML docs in your configuration directory.

Many projects use the DocBook XML DTD to produce documentation.

More examples can be found in the xml.org registry:

http://www.xml.org/xml/registry.jsp