Oh, XML.

I remember when I posted a project on SourceForge, I had to categorize it (aka Trove Software Map). Among the different attributes I filled in, there was the language (used for development). Because my code was heavily based on XML (for describing a MMI, a little bit 'à la' XUL), I intended to declare XML.
But when browsing through the available languages, I noticed that XML was not promoted as "first class" language (neither second class, BTW!). Which scared me (I avoided a heart-attack, at last ;-)
I e-mailed something like "I know that XML is not a 'real' programming language, but due to the increase of its usages in programming technics, why not adding it". No answers (apart from two other 'sourceforgers': one who agreed and another who talked me about the non-LALR(1) aspect of XML, sic).

Things are perhaps changing with the arrival of o:XML. I let you go through an interesting overview on XML.com (don't be put off by the classical 'HelloWorld' code snippet: the power of that language is not there!). Let me quote something really important about the strength of o:XML:

o:XML is a practical language, designed to solve practical problems. For many types of applications it already provides faster, easier, better solutions than any other technology. If you are writing code that processes or produces XML, chances are you will benefit from using o:XML.

In fact you really can avoid the pain of XSLT (sometimes), and use o:XML to script your output XML (like JSP in fact). But that's not the only use case: it is far more productive to write o:XML than Java (or C#) to produce simple XML.

So, when you have to produce XML as an output, you have the choice of XSLT, XQuery AND o:XML. The later shares with XQuery the advantage of being output format-oriented (try to picture what the output will look like by reading an XSLT stylesheet ;-)