MultiMarkdown, or MMD, is a tool that helps you turn minimally marked-up plain text into well-formatted documents, including HTML, PDF, OPML, or OpenDocument. It adds multiple syntax features (tables, footnotes, citations, etc.), in addition to the various output formats. MultiMarkdown started as a Perl script, which was modified from the original Markdown.pl.
Additionally, you can write a MultiMarkdown document in any text editor, on any operating system, and know that it will be compatible with MultiMarkdown on any other operating system and processed into the same output.