Ntp server with drift autogeneration. It can give you the lowest time offsety.
One of the best decentralized messengers for text, audio and video communication between two participants.
An actively developing mesh network, which at this stage is in the form of an overlay on top of the Internet, but allows you to connect devices through any physical connections.
The most popular software for organizing an xmpp server, which is the basis for communication between individuals and entire groups, and there is a lot of customization and many additional modules.
Software that allows you to unite a large number of scattered groups from different communication protocols. It has its own problems, but the developer tries to fix them quickly.