Oomph. XSL-FO. I used that for a custom PDF templating engine once. I think it was called XSL-FOP. Worst XML nightmare that doesn’t lend itself very well to version control. I would prefer markdown any day (or, indeed, using LaTeX with a non-PDF output). Even plain text is preferable.
FO stands for "Formatting Objects" and FOP is "FO Processor", e.g. Apache FOP. The format is not meant to be user-editable, of course, it's an intermediate step, but as a page description language it's very good. I myself use it when converting reStructuredText sources to PDF.
