JAmes' DSSSL Engine - ie, a program that takes a SGML file, looks at a DSSSL stylesheet and produces a formatted output. It was written by James Clark, and it's under BSD-like lisence.

It can spit out SGML (ie, HTML), RTF and TeX.

I must say Jade-generated HTML looks WAY worse than even what Frontpage sometimes generates, but it is remarkably correct what comes to syntax. With DocBook -> HTML conversion, HTML Tidy gives only a few warnings and generates fairly readable source.