Over this past weekend, the XSLT Timer tool has been released on Code Gallery.
This is a very simple tool, that will simply allow you to type / paste in a document and an XSLT stylesheet, and then tell you how long it takes on average to do the transform using the XslCompiledTransform class. Very simple, but handy when trying ad hoc variations on ideas and such.
You might want to look at the XML Resources page for links to other tools as well.
update #1 - adding an actual link to the tool helps 🙂