Induction heating. Started playing with it years and years ago; a bit more recently, I realized there's only maybe three or four people on the internet that are actually doing something on the same scale (upwards of 5kW, suitable for melting and forging metal) and sophistication (i.e., basic things like frequency control and current limiting, let alone UL-required safety features, and common features like timers or profile control). Worldwide industry is in the low $B/year, which sounds like a lot, but maybe three or five companies account for well over 90% of that.
Like power transmission, it's actually a very slow, lumbering business. Tube oscillators from the '80s are still in wide service, and resale from time to time. Yes, '80s -- again, tech moves slowly -- while motor controllers ranging from gen sets to SCR or BJT-fired VFDs have been around since the '60s, those technologies have been only slowly accepted in this field, and at that, mostly for low frequency, high power purposes -- at least, when the hockey puck style devices were utilized. Which again, is around about the time solid state HVDC inverters hit the marketplace.
Tim