A pubnix (public unix) is a multi-user server that acts as a little social-network. Usually, pubnixes offer services such as web, gemini or gopher hosting. Users usually connect to tildes using ssh. They form the backbone of the small-net.
Here's a non-exhaustive list of tildes I'm aware of. Feel free to ping me to add more:
https://rawtext.club/
https://tilde.town