The question is even more ancient now but also even more relevant since much of the mentioned code is several years out of date.
However, I did find a few that still seems current:
Jquery-Markedit - This was forked from wmd-edit quite some time ago and refactored to use jQuery. Seems good at first sight.
EpicEditor - is also still maintained, has a flexible parser and, as you can see below, the author is highly responsive (see below). IT seems to have good documentation as well. Sadly not working with IE9.
MarkdownDeep is a third option that is still current. The interesting point with this one is support for Markdown Extra. Has a dependency on JQuery (actually you can also implement without JQuery). Based on the .NET version so documentation is more aligned to that than the JS version. This also works with IE9. It is very easy to use (with JQuery) & very simple. No significant development happening with this though as far as I can see.
js-markdown-extra is a fairly accurate port of the PHP library and is still under maintenance. It supports Markdown Extra of course.