Email, blogs, forums. Pretty much the whole internet in the 90’s. But then centralisation turned out to be more profitable, so they could hire UX designers and attract all the users...
RetroShare is pretty cool. Initial release was in 2006.
> Retroshare is a free and open-source peer-to-peer communication and file sharing app based on a friend-to-friend network built on GNU Privacy Guard. Optionally, peers may communicate certificates and IP addresses to and from their friends.
> RetroShare is a decentralized, private, secure, cross-platform, communication toolkit. RetroShare provides file sharing, chat, messages, forums, channels and more.