At Yelp, we store nearly all of our data in MySQL. At any given time we're issuing tens of thousands of SQL queries to our database cluster per second, with some individual servers going above the 10k qps mark. Our database cluster consists of billions of rows. In response to a lot of different problems we've had to optimize the snot out of our MySQL installation, and we've learned some interesting things along the way.
A colleague and I recently gave a presentation to some of our coworkers, titled MySQL Minutiae & InnoDB Internals. The talk covered some good background knowledge that every developer should have about MySQL (transaction isolation levels, replication, etc.) and some more advanced topics such as InnoDB locking semantics.
There's some good stuff in the talk for people of almost all skill levels, but to whet your appetite I'm going to dive into an interesting deadlock example.