I have been a Wikipedia contributor since the beginning January 2005, so my first exposure to anything Markdown-like dates back to that time. There are plenty of similarities in the structure between MediaWiki formatting and Markdown.
Years later in 2008, I found my first exposure to the Twiki syntax because of its usage in a corporate environment, though I'm far less a fan of that syntax.
Notes
I'm also not a fan of Setext-style headers, which go like the code below:
First-level heading
===================
You can test what this looks like by copy and pasting everything here into the Markdown conversion tool. Watch out for trailing whitespaces, as
tags are represented by double spaces at the end of the line. (I have mixed feelings about that one too.)