Table tennis robots
In the Beginning, God (I mean Sitco, I think they were first) created table tennis robots. They had many problems. They either hit to one spot on the table over and over, or they sprayed the ball about randomly, so you couldn't really do many table tennis drills with them. They were either set to heavy topspin or heavy backspin; there was no in between. And the ball was shot at you by spinning disks instead of coming off a paddle, like in a real game, so you didn't learn to read the ball off a racket. (There were other problems early on, such as catching the balls, recycling them, consistency, etc., but these problems were all worked out long ago.) Fixing these problems were, to me, the three holy grails of table tennis robots.
Many of the modern robots are now programmable so you can actually do real drills with them - in fact, just about any drill you can do with a partner, you can do with these robots. Plus you now have more control over the degree of spin. So they are starting to look like more than glorified toys with nets that could catch the ball for you when you practiced serves, which was my primary use for them for many years. (They were also good for group training with beginners, who were fascinated by them, and allowed you to put 1-3 players rotating on the robot while they worked on basic shots.)
So robots are now much, Much, MUCH better than before, and you can actually get a great workout with them. They are good training for players, especially the ones that have built-in and programmable drills you can choose from. I keep waiting for them to break out among the general public as fitness devices, since anyone can do footwork drills on them, moving side to side and getting in shape, even if you miss many of the shots at first.