The high-toss serve used to be one of the most popular serves at high-level table tennis, and pretty common at the intermediate level as well. There are still plenty of players who use it, but it is not as common as before. Why is this? First, you should understand what the serve is, and its advantages and disadvantages.
A high-toss serve is just that - a serve where you toss the ball high into the air, often ten to fifteen feet up. They are the most spectacular of serves. They are also the hardest to control. If you want to learn a high-toss serve, watch how the top players do it, and practice a LOT. Here's a video on the high-toss serve (2:22), featuring Japanese star Jun Mizutani.
Most players below the advanced levels can't really control a high-toss serve, and so the serve tends to be easy to attack once you get used to it. But because the serve is "different" many players have trouble with them, even if they aren't done very well. Most high-toss serves are done with a forehand pendulum serve. (For a righty, this means the racket is tip-down, and moves from right to left. For a reverse pendulum serve, the racket would move left to right.)
Advantages of a High Toss Serve
From the advantages, you see that you can get more spin on the ball. This is especially effective when going for an extremely heavy backspin serve, which opponents will often put in the net since they aren't used to such heavy backspin - but only if you perfect the serve. It's not easy learning to throw the ball up way in the air and just graze it as it comes down! But if you learn to do so, the serve can be highly effective - especially if you also learn to vary the spin, with varying degrees of backspin, sidespin, and corkscrewspin, as well as no-spin serves that look like backspin. (Not sure what corkscrewspin is, or have other questions on spin? See my article "Everything You Ever Wanted to Know About Spin - But Were Afraid to Ask.")
However, as you can see there are also disadvantages, and they outnumber the advantages 4-2. This doesn't mean you shouldn't high-toss; it means you have a bit more practice to do. With practice, you can negate the first three disadvantages as you learn to control the depth and height, and perhaps learn to do a reverse pendulum serve with a high toss. (Some players simply use a lower toss for the reverse pendulum serve, and with a high toss only do normal pendulum serves. Not sure what a reverse pendulum serve is? Here's a short article on it, which includes a link to a short video which shows both a regular and reverse pendulum serve.)
The key problem with a high-toss serve is the speed at which the ball falls at contact, which is both the strength and weakness of the serve. While it does allow more spin, the faster-moving ball means you have less time for deceptive motion as the ball goes by, as well as a loss of control. Most players find they get more deception and control with a shorter toss, which is why most serves toss the ball perhaps head high. However, the high-toss serve is still a great weapon to have, either as a front-line serve used over and over, or as a variation to throw an opponent off, which is how I use it. Many players will toss the ball head high over and over, and then, perhaps several times a game, using the same motion, they'll suddenly go for a high-toss, which often leads to a completely befuddled opponent who simply isn't ready for the extra spin (especially backspin) of a properly executed high-toss serve.
Another advantage of the high-toss serve is that since fewer players are using it than before, players are less used to it, and so have more trouble with them. So if you are looking for an extra weapon for your serving arsenal, get high with the high-toss serve.