MultiMarkdown
Expanded perl version of John Gruber's original Markdown
...Unlike standard Markdown, MultiMarkdown adds capabilities for tables, footnotes, citations, cross-references, and metadata, making it especially valuable for longer and more detailed documents. Its design philosophy is to maintain Markdown’s ease of use while expanding its ability to handle professional publishing needs. Although this version is no longer under active development since MultiMarkdown 3, it remains a reference implementation for earlier enhancements to Markdown.