I have been using the XSelerator for almost 10 years and have never found a better tool XSLT IDE.
XSelerator's most valuable features are:
XSLT Debugger for MSXML3 and MSXML4.
Intellisense (prompting the xslt instructions, their attributes and the enumeration of possible attribute values.
Dynamic intellisense: prompting with template names or with modes or with parameter names.
XSLT 2.0 intellisense.
One-click invocation of many XSLT processors.
One can easily add a new XSLT processor to XSelerator.