http://ettext.taint.org/ (only produces HTML)
http://www.prefab.com/ssl/notagsmarkup.html
http://txt2html.sourceforge.net/
http://www.xmlmind.com/aptconvert.html
http://www.yaml.org/ -- general markup, not just formatting
http://www.perldoc.com/perl5.8.0/pod/perlpodspec.html
http://docutils.sourceforge.net/mirror/setext.html (dead?)
http://docutils.sourceforge.net/rst.html
http://dev.zope.org/Members/jim/StructuredTextWiki/FrontPage/
http://www.maplefish.com/todd/aft.html
http://www.cit.uws.edu.au/~blilburn/sdoc/sdoc-2.1.html

http://www.usinglinux.org/textproc/