Apps that let you act on Ethereum without revealing who you are.
The idea: You deposit into a shared pool and receive a secret note. Later you prove, in zero knowledge, that you own some unspent note in the pool's Merkle tree without saying which one, and a "nullifier" derived from the note stops you from spending it twice.
Why it matters: They rely on zero-knowledge proofs on BN254 and the Poseidon hash.
On the map: Semaphore, Privacy Pools: Groth16 + Poseidon on BN254
Applications that let users act without revealing who they are, such as Semaphore (anonymous signaling and voting)[1] and Privacy Pools (private withdrawals with association sets)[2].
They use Groth16 proofs on BN254 with the Poseidon hash inside the circuits[3], so they inherit BN254's assumptions, the trusted setup and Poseidon's security.
If any of these breaks, this breaks too.
If this breaks, these break with it.
Nothing on the map depends on it: it is an end point, something people use directly.