The DocBook DTD itself is maintained by the DocBook Technical Committee, headed by Norman Walsh. Norm is the principal author of the DocBook stylesheets, a man who has focused remarkable energy and talent over many years on the extremely complex problems DocBook addresses. He is as universally respected in the DocBook community as Linus Torvalds is in the Linux world.
libxslt is a C library that interprets XSLT, applying stylesheets to XML documents. It includes a wrapper program, xsltproc, that can be used as an XML formatter. The code was written by Daniel Veillard under the auspices of the GNOME project, but does not require any GNOME code to run. I hear it's blazingly fast compared to the Java alternatives, not a surprising claim.
xmlto is the user interface of the XML toolchain that Red Hat ships. It's written and maintained by Tim Waugh.
Saxon and Xalan are Java programs that interpret XSLT. Saxon seems to be designed to work under Windows. Xalan is part of the XML Apache project and native to Linux and BSD; it's designed to work with FOP.
FOP translates XML Formatting Objects to PDF. It is part of the Apache XML project and is designed to work with Xalan.