The number one feature I've found missing in MarsEdit is the ability, not just to preview draft posts in Markdown, but actually to convert them from Markdown to HTML prior to posting them. Admittedly a number of blogging platforms now allow you natively to post in Markdown, but in WordPress it can only currently be done using the Markdown PHP plugin.
After thinking about it, I realised this issue was easily solved by creating a separate automated process to carry out the conversion and making it accessible within the two above applications. To do this, I decided to use Automator to create an OS X Menu Service that will convert selected text in any application from Fletcher Penney’s excellent MultiMarkDown, which is a fork of MarkDown, to HTML and copy the result to the keyboard. This is almost childishly simple. You open Automator and choose the ‘Service’ template for your workflow.
In the workflow pane, you check the ‘Replaces selected text’ box and add the conversion script of your choice choosing the ‘Run Shell Script’ action. You then add the script of your choice, with the path in which you’ve stored it, to the workflow pane. I also chose to add a command to pass on the resulting HTML to the clipboard.
I can either use this command in my text editor, WriteRoom (or any application), then open MarsEdit and paste the resulting HTML in the editing pane. Or I can paste the MultiMarkDown text into MarsEdit, which has a Markdown preview filter, and convert the text prior to posting it.