A closer look at transformative tools

XSLT XSLT is in some regards the 'Ur-transformation' tool. While it was built for documents, it's been reapplied widely to data processing.
Omnimark Came before XSLT, and supports a wider variety of processing types, but is proprietary. Was free for a while, but no longer.
Custom-built text processing The 'Desperate Perl Hacker' has been with us from the beginning, and there's all kinds of useful work you can do with regular expressions that's difficult or impossible to do with XSLT.
Custom-built XML processing (SAX filters, DOM processors) The sky's the limit, though reusability is tricky and complexity can be high. These tools can also be used to create XML views of non-XML information.