Jami

Jami is a communication program that works without a central server. Rather than registering with a service that stores and routes messages, an account is a cryptographic key pair generated on the device itself; contacts are located through a distributed hash table, and calls and messages travel directly between participants when the network permits.

It handles audio and video calls, group conversations, messaging, file transfer and screen sharing. Conversations synchronise between the devices linked to one account instead of being held on a server, which is why a message sent while the other side is offline arrives once that device returns. Jami can also act as a SIP client, registering with a conventional SIP provider or an office PBX alongside its own accounts.

The absence of a server is the part to plan around. There is no password reset: the account exists only as key material on the devices, and losing every linked device without an exported backup loses the identity with it. Direct connections are also harder behind restrictive NAT, where traffic has to go through a relay.