Modern Markdown converters have an annoying habit of not offering a typography conversion layer (such as SmartyPants) or similar functionality.
- The original Markdown Dingus includes a SmartyPants layer, but it doesn't handle em-dashes. Also, the original Markdown doesn't support the use of footnotes and other newer conveniences.
- The PHP Markdown Extra dingus includes a SmartyPants layer, but some months ago the author removed the option to use Markdown Extra (which supports footnotes) and Smartypants at the same time. (Why?!) The workaround is to run the text through twice: once with the Smartypants filter, then copy the new typographied Markdown text into the source box and wash it through the Markdown Extra filter (the order is important).
- WriteMonkey includes an extremely handy Copy-as-HTML feature which converts your plain text (as MultiMarkdown) to HTML and places the result in the clipboard. Again, though, no typography layer is included or supported. Writemonkey also has a SmartyPants plugin, but it must be run as a separate step. It also alters the original text rather than presenting the converted result separately.
- Drafts (online text editor similar to Editorially but actually predates it) -- it can convert your markdown text to HTML, but weirdly translates your plain-quotes into HTML entities of plain quotes (not curly ones). It doesn't touch ellipses or dashes.