With a little file creation trickery, it's really quite easy to build an ActivityPub server.
Guppe brings social groups to the fediverse — making it easy to connect and meet new people based on shared interests without the manipulation of your attention to maximize ad revenue nor the walled garden lock-in of capitalist social media. Guppe groups look like regular users you can interact with using your existing account on any ActivityPub service, but they automatically share anything you send them with all of their followers.
This page even has a list of groups to get you started!
GoToSocial provides a lightweight, customizable, and safety-focused entryway into the Fediverse, and is comparable to (but distinct from) existing projects such as Mastodon, Pleroma, Friendica, and PixelFed.
With GoToSocial, you can keep in touch with your friends, post, read, and share images and articles, without being tracked or advertised to. Because GoToSocial uses the ActivityPub protocol, you can hang out not just with people on your home server, but with people all over the Fediverse, seamlessly.
Full support for modern, elegant apps like Tusky and Pinafore.
BookWyrm is a platform for social reading! You can use it to track what you're reading, review books, and follow your friends. It isn't primarily meant for cataloguing or as a datasource for books, but it does do both of those things to some degree. With ActivityPub, it inter-operates with different instances of BookWyrm, and other ActivityPub compliant services, like Mastodon and Pixelfed. This means you can run an instance for your book club, and still follow your friend who posts on a server devoted to 20th century Russian speculative fiction. It also means that your friend on mastodon can read and comment on a book review that you post on your BookWyrm instance.
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/
Epicyon is an AGPL licensed ActivityPub protocol compliant federated social network server suitable for hosting a small number of accounts on low power systems requiring minimal maintenance, such as single board computers. It's the ActivityPub equivalent of an email server, storing posts as human readable JSON on file, rather than in a database. It also uses only a small amount of RAM.
Python, HTML+CSS. Almost no JS is used.
Has pretty much all of the features you'd expect. Has a calendar feature for local users. Has bookmarking of specific posts.
Put the full RSS feed URL in here, and pick a username for the account that will track the feed.
ActivityPub-PHP is a library that embeds a full ActivityPub server into any PHP project. It works with any SQL database and any web framework. At a high level, it provides a request handler that you can route ActivityPub requests to which will take care of persisting the received activity, performing any necessary side effects, and delivering the activity to other federated servers. It also provides a PHP API to create and manage actors and activities.
A federated microblogging application written in node.js. Replaces Twitter. Implements ActivityPub so it's already part of the Fediverse.