It seems to me, that one of the most important aspects of software development is one that doesn't get a great amount of focus. Debugging. Sure, it's mentioned here and there, but, for example, first year students aren't even taught about the command line Java debugger.
So, I believe this video of Stuart Halloway, "Debugging with the Scientific Method" is required viewing. Of course, it's not just debugging, but any sort of performance or work on a website or application. Take stackoverflow for example, it's a popular site and hosted on their own servers. I have been reading lately on their setup and the monitoring they do, not only for uptime, but for performance.
For example, they use HAProxy to load balance to their web tier servers, obviously not unusual, that's what HAProxy is for. But, they also have these proxies capture and filter performance data from their application via headers in the HTTP response. It's probably something that everyone does, but to be honest, I've never come across any mention of this trick. (There's also their miniprofiler tool, which I'm using a variant of).
Given how little debugging is taught in university (well, my university) I can't judge on how common and detailed this sort of performance measurement is. I suspect that it might not be very common, so could be an interesting area for me to focus on.