Pleroma is a free, federated social networking server built on open protocols. It is compatible with GNU Social, Mastodon, and many other ActivityPub implementations.
The project consists of several components: Pleroma is the server implementation, and comes bundled with PleromaFE, the default frontend. Other useful utilities are also provided, such as an ActivityPub relay.
High performance, low latency, you can even run it on a RasPi (and many busy Pleroma instances are!)
Written in Elixir and Phoenix, with Postgres as its back-end.
Source code: https://git.pleroma.social/pleroma/pleroma/