matrix.org is federated rather than p2p, but it's e2e implementation has some very nice features (group chat, multiple devices, forward secrecy, etc.).
Two very cool projects coming up are Briar and Cwtch. Both are P2P, E2E, and metadata-resistant over tor
Briar already has a stable release but I consider it not very viable for most communications because it doesn't yet have the ability to remotely add contacts (you need to scan the peer's QR code from their phone). But they're working on implementing that.
Cwtch isn't out yet, but it is going to implement many of the same features as Briar, with the added perks of having desktop and mobile clients with syncing between the two. It accomplishes this through an untrusted federation of Cwtch servers running over tor to store offline content (when a peer is down) and group chats.
As someone else mentioned Matrix is also a thing. It's not P2P and E2E is still in beta, but it also accomplishes different things compared to Tox et. al. like federating different chat protocols like IRC.
There are a large number of them, but most are sadly either abandonware or vaporware. The closest thing I feel we have right now is really matrix.org. User identities are federated unfortunately, but this makes things that are going to be a problem on any fully decentralized system, like mobile push notifications, simpler.
If you don't mind proprietary solutions, Firechat [1] is one of them. It allows chatting over a mesh network using WiFi or Bluetooth even with no Internet connection.
