bignosePre-dating Markdown and more general-purpose by design, the reStructuredText format is part of #Docutils; greatly expanded on by the #SphinxDoc project.
Both are #FreeSoftware and exemplars of practical, community design projects.
I'm #ThursdayThankful for #reST: a sensible, readable plain-text markup format with thoughtful design to make it unobtrusive and semantically powerful.