Fast and Deep Serves
I've been teaching this a lot recently. These are rarely front-line serves as even intermediate players have little trouble attacking them if you use them too often. However, they are a great variation to spin serves, and if used a few times each game will often catch the opponent off guard. I probably use them more than most both because I'm confident I can pick just the right time (you get a sense for that with experience), and because I spent so much time practicing this in my early years that I have very good fast and deep serves.
Before we go on, isn't fast and deep serves rather redundant? If the serve is fast, it's obviously deep, right? And yet it's part of our lexicon that we call these serve fast and deep serves rather than just fast serves.
Here's a tutorial (2:51) from PingSkills on fast and deep serves (okay, they actually call them "fast and long serves," those Aussies), which covers the topic pretty well. Note the emphasis on having the first bounce hit as close to your end-line as possible, to maximize the time the ball has to drop over the table - this is extremely important. Putting a target on your own side of the table to see if you are hitting the ball near your end-line is a great way of teaching this; I also use that method. Equally important is having a low contact point. (Most players contact the ball too high on all serves. It's a common problem even at higher levels, and many don't even realize this, and so their serves aren't as low as they could be, making things easier for the receiver, whether they attack or control the serve back.)