Rapid prototyping in general, and additive manufacturing (3D printing) specifically. I used to spend a lot of time prototyping by hand with foam, paper, tape, wood, clay, etc. We would have “arts and crafts week” at work when it was time to step out of CAD and make concept models of some new design. Occasionally some VP would walk by and wonder why the company was paying a bunch of salaried engineers to stick cardboard pieces together with hot glue. Getting decent parts often meant spending thousands of dollars on molding tools to make cast urethane prototypes.
Now I can walk down the hall to one of my 3D printers and get beautiful parts overnight. And there are a ton of companies offering rapid machining and sheet metal forming services. Getting design feedback and finding mistakes is so much faster and easier when I can go from 3D on a screen to real parts in my hand in a few days.