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.