MultiMarkdown
Expanded perl version of John Gruber's original Markdown
...The project allows users to write documents in a human-readable plain text format and convert them into structured HTML or other formats. 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.