Stack Exchange's markdown is just a variation of standard Markdown, so the MultiMarkdown editor may be enough of what you want.
Other options:
MarkdownPad - a "fully featured Markdown editor with live preview". Developed by a SO aficionado.
Emacs Markdown Mode - a plugin for the common programming editor.
See, also, "Are there any text editors which are Markdown aware and provide spell checking features?""Are there any text editors which are Markdown aware and provide spell checking features?"
You can always make your own word-processor, that uses Stack Exchange's exact syntax, by wrapping a GUI around Markdown Sharp, which is Stack Exchange's implementation of its custom markdown.