Daring Fireball: Markdown - the original specification of Markdown by John Gruber.
CommonMark - a strongly defined, highly compatible specification of Markdown.
GitHub Flavored Markdown - a superset of the CommonMark spec containing extensions for tables, task list items, strikethrough, autolinks, and disallowed raw HTML.
Markdown Content Blocks - adds support for file transclusion (images, text files, and CSV tables) with captions.
Markdown Extra - an extension to PHP Markdown that implements some features that are not available with the regular Markdown syntax.
MultiMarkdown - a superset of Markdown, adding support for a large number of Markdown extensions and output formats besides HTML.
Pandoc’s Markdown - another superset of Markdown from the author of CommonMark, adding support for many new features and designed for multiple output formats.
GitLab Flavored Markdown - similar to GitHub Flavoured Markdown, but adding support for a number of new features such as video, Mermaid diagrams, and GitLab-specific references.
Markua - called “Markdown for books”, Markua is the successor to Leanpub Flavored Markdown that maps some Markdown syntax to book concepts, while adding new syntax and concepts.