As far as I know there really isn't any other browser-based editor for Markdown, at least none as extensive as the WMD editor.
Showdown is a Markdown converter in JS, which forms the basis for the HTML preview of WMD. They're both made by http://attacklab.net/.
And as far as I know there haven't been any big complaints about both (at least not on the Markdown mailing list). So go for it.
