One of the coolest things about the OPML Editor is the Map A Domain feature, through which the community server that Dave set up (running on the Frontier kernel) can translate an OPML file to a tree based web site. For an example, see my mapped domain: opml.yabfog.com.
But this is just a prelude to the exciting part: I wrote an XSL stylesheet that translates an XBEL bookmark file (such as that produced by the Firefox web browser*) into OPML. I initially tried to do this (and succeeded) with PHP, but that was a royal pain (and trouble-prone because I was treating the XML as a text format). I had seen Joshua Allen's solution for applying XSL Transformations to OPML, but it worked in the wrong direction. So I taught myself how to use XSL.
I am really pleased with the results.
And this XSL stylesheet transforms XBEL to XHTML.
Here is the resulting OPML output.