If it's free and open source and it's also software, it can be discussed here. Subcommunity of Technology.
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Yggsrqsil network I2P Freenet https://en.wikipedia.org/wiki/B.A.T.M.A.N. IPFS SSB / Manyver.se / PZP Briar Project Tailscale Syncthing Localsend
There are lots of others, but most either shut down, got turned into scams, or were always scams.
There was one chat app I remember, I think it was called firechat or something. They ended up removing all the P2P features from it and removed all references from their advertising.
Monero - private, permissionless, decentralized, p2p, digital cash system.
Reticulum - cryptography-based networking stack designed to build secure, decentralized, and resilient local or wide-area networks using readily available hardware. It operates independently of traditional internet protocols (like IP), enabling communication under adverse conditions such as high latency and extremely low bandwidth with:
Basically internet 2.0
Arweave - private, permissionless, decentralized, p2p, permanent data storage/server.
I have a strange love in my heart for IPFS. I can't justify it, but it's there. Beautiful atoms... such promise...
Same
Croc file share, uses a relay to find each other https://github.com/schollz/croc
Session Messenger, uses a tor style server network https://github.com/session-foundation/session-android
Tox is interesting, though potentially less secure than Briar as they rolled their own encryption and have not been audited (afaik).
there is gnu jami - messenger and video/audio calls