Newly released: Xuriella XSLT, the first implementation of XSLT written entirely in Common Lisp.
Based on Plexippus XPath, Xuriella is an implementation of XSLT 1.0, written by Ivan Shvedunov and me.
Xuriella is quite complete and correct -- we run the official testsuite, with more than 95% of tests currently passing.
One advantage of a pure-Lisp implementation is that extension elements (as well as XPath extensions) can be defined easily.
That's a huge plus because XSLT itself is a very specialized programming language -- it excels at XML/HTML generation and transformation only. Being able to write custom extensions in Lisp helps with any non-XML-ish parts of the task which XSLT itself might not handle conveniently.
If you just want to try applying stylesheets, there are only two functions you need to know about: parse-stylesheet and apply-stylesheet.
For details about these functions (and all others, including those for extensions), refer to the API documentation.
The example uses Hunchentoot and Xuriella with XSLT as a template language in a simple directory listing request.