Ask HN: What new development technology in the last 3 years is worth learning?

For the last 3-4 years I've been working at a super small startup doing Rails, and basically just using my existing technical knowledge and avoiding the learning cost of new technologies. I know React has gained a lot of popularity -- what else is worth looking at as I take my head out of the sand?

Not limited to web programming; OpenCV and other technologies are neat too!