What are the useful products where ZK provided a necessary unlock?

Working on building zkTLS infra/products. A ruthless empiricist and meme connoisseur.

http://opacity.network

Can digital signatures be seen as a kind of zero knowledge proofs? I can prove I have the private key without revealing it. 
If yes, so many applications

Building things, like https://monique.app •
Also web3 tech lead at Ubisoft

A lot of venture capital has made its way into zk, so I’m actually asking how many products has that money brought to market