Ask HN: Small but rapidly growing categories?

I'm wondering if anyone has some good examples of emerging industries (or segments of those industries) that are still under a billion dollars in revenue per year, but are experiencing double digit growth. I'll start with an example - Deception Technology https://en.wikipedia.org/wiki/Deception_technology - according to the wikipedia article, some analysts put it at under a billion dollars and show it growing. This is the space I'm in with my current startup (https://www.tamarin.us) What other "early adopter" categories like these are out there?