Stack Exchange's markdown is just a variation of [standard Markdown][1], so the [*MultiMarkdown* editor][2] may be enough of what you want.

Other options:

- [MarkdownPad][3] - a "fully featured Markdown editor with live preview".  [Developed by a SO aficionado][4]. 

- [Emacs Markdown Mode][5] - a plugin for the common programming editor.

- See, also, ["Are there any text editors which are Markdown aware and provide spell checking features?"][6]

- You can always make your own word-processor, that uses Stack Exchange's exact syntax, by wrapping a GUI around [Markdown Sharp][7], which is Stack Exchange's implementation of its custom markdown.


  [1]: http://daringfireball.net/projects/markdown/
  [2]: http://fletcherpenney.net/multimarkdown/
  [3]: http://markdownpad.com/
  [4]: https://stackoverflow.com/a/7924828/331508
  [5]: http://jblevins.org/projects/markdown-mode/
  [6]: https://web.archive.org/web/20150928070650/http://superuser.com/questions/235519/are-there-any-text-editors-which-are-markdown-aware-and-provide-spell-checking-f
  [7]: http://code.google.com/p/markdownsharp/