Here are some sites that document how to do different things (since I always forget some of them):

• Daring Fireball Markdown
• MUD Markdown Cheat Sheet
• Markdown Help
• Markdown Cheatsheet
• Markdown Cheat Sheet

All are good and some have things others don't.

Some of the features I tend to want to use and forget are:

• footnotes
• picture links
• definitions

There are others, but with these sites, I can easily find them.