What tool do you use for testing and developing XPath expressions?

Answer 1

oXygen XML

Not free, but worth it if you do any serious XPath/XSLT development.

Answer 2

If you use Notepad++, there's a plugin for that called XML tools.
http://sourceforge.net/apps/mediawiki/notepad-plus/index.php?title=Plugin_Central

There's also Sketchpath which is more complete.

And this made me realize this is a duplicate of
http://stackoverflow.com/questions/801858/tool-utility-for-checking-debugging-xpath-queries/802300#802300

Answer 3

I use the Xpath Checker plugin for firefox. Works quite nicely, but I wouldn't classify it as a "serious development tool". But very handy for testing xpath expressions.