Lately, I have been working on markdown-mode, a major mode for editing Markdown files in GNU Emacs. The latest version (revision 1.3) provides syntax highlighting, insertion commands for all basic HTML elements, and preview commands for viewing the resulting HTML in a new buffer of an external browser.
I had originally planned for it to support itex, a TeX-based markup language for mathematics, so that I could edit pages on Instiki MML+, the MathML-enabled wiki software that I use on xbeta. Implementing that turned out to be more difficult than I expected since the multiline font lock support in Emacs doesn’t work very well. Enabling the package to parse LaTeX or itex will require something more complex than I have time to implement right now. One option might be to instead implement the Markdown syntax highlighting and editing commands as a minor mode and then run it on top of AucTeX.