Cwtch, a decentralized peer-to-peer messaging app.
It lets people message each other without a central server, and is designed to hide not just message content but the surrounding data about who talks to whom.
It runs on Tor hidden services and is built to resist metadata collection, acting as a local mesh-network alternative to routing everything through the global internet.
It's an example of communication tools designed to be resistant to surveillance by keeping data off central servers.