Learning Tool for XPath?

Hi there,

I'm impressed by the power and flexibility of XPath expressions... and 
overwhelmed too. While Michael Kay's XLST reference book on my desk 
already shows strains from heavy usage, I'm still looking for a good 
learning tool for xpath. Something like a visual expression builder that 
helps with the axis syntax and shows xpath expressions between two nodes 
or so.
Are tools like this out there?
;-) stw