Takahē is a new ActivityPub server, designed for efficient use on small- to medium-size installations, and which allows you to host multiple domains on the same infrastructure.
It’s currently in an alpha state, meaning things work (and it will talk ActivityPub to Mastodon and other servers), but there’s still a lot of features left to add.
The flagship instance can be found at takahe.social, and if you’re interested, you can also read more about the project, or read some of Andrew’s blogs on building it.
Main website: https://jointakahe.org/
Documentation: Takahē documentation
Code: GitHub - jointakahe/takahe: An ActivityPub/Fediverse server