The first project I worked in applied ZK, hosted at https://heyanon.xyz.

It allowed users with various ETH reputation like owning an NFT or participating in an on-chain event to anonymously post to different Twitter feeds. It was the first app to productionize 0xPARC’s circom-ecdsa library in a web application, using a variety of tricks from double-blind, Blaine, and Nalin.