XML / XSL Support
The TeXMLMate Plug-in adds an XML parsing palette to the popular TextMate text editor for Mac OS X. While editing an XML (or XHTML) document in TextMate, you can open the TeXMLMate palette to conveniently check your document for well-formedness or validity against a DTD, W3C XML Schema, RELAX NG schema, or Schematron schema
Latest version also allows you to run XPath expressions against your document and see the selected nodes.
The TeXSLMate Plug-in adds an XSLT/XQuery debugging palette to the popular TextMate text editor for Mac OS X. While editing an XSTL stylesheet or XQuery program in TextMate, you can open the TeXSLMate palette to conveniently transform/execute the stylesheet/query against any source file and see different views of the result as well as console error messages
His instructions tells you to quit TextMate and copy the plug-in to the appropriate destination, but of course you can also just double-click it and let TextMate take care of it! :)
Andreas Schöller also posted a bundle with XSLT snippets.