Apache's FOP Project - Open source XSL-FO implementation; converts XSL-FO to PDF and other formats.
html2fo - Generates valid FO documents from almost any (even badly formated) HTML file. Freeware.
PassiveTeX - PassiveTeX is a library of TeX macros which can be used to process an XML document which results from an XSL transformation to formatting objects. PassiveTeX provides a rapid development environment for experimenting with XSL FO.
TeXML - Although this is not an XSL-FO tool, it is used in typesetting. Given that it was posted in 1998, it would probably be overtaken by XSL-FO now. The materials also include an example of MathML markup transformed into TeXML, with PostScript generated using TeX.
