The Markdown Mark
I'm making something that uses Markdown, and there's currently no great universal symbol for identifying Markdown support. So I created one.
You should use this mark to identify user input areas which support Markdown-compiled HTML output or to identify general Markdown support. See the “Usage” section in the Markdown Mark documentation on Github.
You can find the specifications and a PSD with vector shapes on Github, at dcurtis/markdown-mark.