What advanced existing features of SE/MY would a more experienced user want to know?

Question

Two months after I joined MY, I wrote a beginner's guide. Some have found it useful and have been pointing new users to it in their welcome message.

Two and a half years later I have learned a few more tricks, but recently realized there were more I still didn't know.

So I thought of a small list of links to such advanced features which might be useful for more experienced users. I invite you to expand the answer below or, even better, add yours.

Answer 1

• HTML tags allowed in messages (e.g., br for line breaks, sup for smaller characters)
• Advanced search options (e.g., is:question to see only questions)
• Hidden features of StackExchange (e.g., this handy way to resize pictures)
• "Magic links" in comments (I particularly like [edit] -- thanks to Monica Cellio and DoubleAA)
• Recent feature changes to Stack Exchange
• MY glossary to link words to that require definitions
• Mi Yodeya Referencer (userscript) to generate links to common sources
• Markdown editing:
  • Using the buttons at the top of post-editing boxes.
  • Using the "?" button on the top-right to get instructions for using the buttons.
  • Full Markdown Help page
• List of handy websites for linking directly to sources: Sefaria, Chabad, HebrewBooks Beta, AlHatorah, Mechon Mamre, Wikisource.
• SEDE Tutorial