What is Markdown
Markdown is a lightweight markup language, originally created by John Gruber and Aaron Swartz, allowing people "to write using an easy-to-read, easy-to-write plain text format, then convert it to structurally valid XHTML (or HTML)".
The language takes many cues from existing conventions for marking up plain text in email. In other words, Markdown is a text-to-HTML conversion tool (for web writers).
Markdown is also a Perl script written by Gruber, Markdown.pl, which converts marked-up text input to valid, well-formed XHTML or HTML and replaces left-pointing angle brackets ('<') and ampersands with their corresponding character entity references. It can be used as a standalone script, as a plugin for Blosxom or Movable Type, or as a text filter for BBEdit.
Markdown has since been re-implemented by others as a Perl module available on CPAN (Text::Markdown), and in a variety of other programming languages. It is distributed under a BSD-style license and is included with, or available as a plugin for, several content-management systems.
retext => Ubuntu PPA ppa:mitya57/ppa
vim / Sublime Text 2 / uberwriter
Mac OS X
Markdown Pro / vim / Sublime Text 2
MarkdownPad / vim / Sublime Text 2