For use on the web you'll need a server license, though, which is $3800 for one machine. Fine if it's a business with revenue, but pretty steep for side projects or free services.
(There's also a free version for non-commercial users, but it's really only usable for evaluation, because it adds a PrinceXML logo to the generated PDFs.)
We offer our database publishing software as open source (http://speedata.github.io/publisher/index.html). It is used in many commercial environments as a better alternative to XSL-FO. Our software allows much more flexible layouts. That said: english documentation is still behind and we are lacking nice show cases. If you need more info, just contact me.
IIRC it's one of the few big applications written with Mercury functional logic language.