The following is a collection of links to macro and mark-up languages, their associated processors, and several mark-up converters. The links are to Google searches, as I am lazy and the items are in varying states of being supported. One missing from the list below is zoem.

Although I have not used it, the gema language seems well thought-out and powerful. It transforms text by applying pattern transformations, so it is not tied to a single syntax.

Christopher Browne has a section on macro languages: http://linuxfinances.info/info/macros.html

There is a section on converting to/from HTML at http://www.hypernews.org/HyperNews/get/www/html/converters.html

You might also be interested in these Freshmeat tags: text-processing, Markup, macro+text.

The Debian text section http://packages.debian.org/unstable/text/ has many interesting entries, and so has the OpenBSD Ports textproc section http://openports.se/textproc/.

The following wikipedia page links to this humble page, presently (and unrelatedly) needs cleanup, and mainly has a comparison of various fairly free-style mark-up languages. For example, *bold* is typically bold, /italic/ may be italic, [[...]] denotes a hyperlink, =...=, ==...== denote first and second-level headings, and so on. I do not very much like this style of mark-up as it seems fragile. For example, white-space is significant in many different ways. It introduces a large amount of syntax and hard-wired conventions, has poor escape mechanisms, usually lacks the power of macros and is hard to extend. Anyway, the page is this.