XSLT
Been playing with XSLT a bit recently. It is quite a nice transformation language. I have been porting the gtk-doc DocBook -> HTML conversion program to use xsltproc (with a customisation layer over Norman Walsh's XSL stylesheets), rather than Jade (with a customisation layer over his DSSSL stylesheets). Took a little while to learn what I needed, but the end result looked fairly elegant.
While working on the customisation layer, I even found a simple bug in the base stylesheets.