[Cryptography] Where are zero knowledge proofs actually used in the real world?

[Alfie John]

Zcash are using them to anonymise payments:

  https://z.cash/tech.html

  Our zero-knowledge proving scheme makes it possible to hide values
  and other identifying information from the public. Using advanced
  cryptographic techniques, users simply prove that the values balance
  and that they are authorized to spend such value. (And that they have
  not spent it before.)

Alfie