Never got into the XML and stuff...I probably should... Has a lot to do with motive I guess.
XML, and especially XSLT (XML's version of style sheets), are very interesting. Another platform of note is
Apache::AxKit which is an "XML Application Server" built on Apache and mod_perl. I've built micro Free Republics in both Apache::AxKit and Apache::PageKit. Both are incredibly extensible, though I tend to favor Apache::PageKit over AxKit because, while XML and friends are interesting, they scare the hell out of me. :-)
(You don't have to know much about XML to use Apache::PageKit, you can use simple templates and good ol' camel grease to pump out pages.)