Gathering links here as I begin the judging process.
by @jacobswain
http://www.markdown-tutorial.com/
by @agea
http://agea.github.io/tutorial.md
by @Tyler_Long
http://tylingsoft.github.io/tutorial.md/
By Sean Francis N. Ballais (via email)
http://seanballais.github.io/Marky/
By Joshua Gross (via email)
http://thisismarkdown.com/
By pkarthikr (via discourse.codinghorror)
http://pkarthik.in/learning-markdown
https://github.com/pkarthikr/learning-markdown
By eh3rrera (via discourse.codinghorror)
http://eh3rrera.github.io/markdowntutorial/
By JosephErnest (via discourse.codinghorror)
http://thisisvoid.org/markdown