Note: most of these projects include both a back-end and a web client, but this list can also include projects that are only a back-end. Projects that are only a web app, for use with an existing back-end, will go on the client watchlist here.
Friendica (see above) has an events engine but there are conflicting reports about whether or not federation of events over AP is working yet. NextCloud federated events are in the work but this may or may not be AP-compatible.
✔pubgate - "Asyncronous Lightweight ActivityPub API ... Based on little-boxes. Implements both the client-to-server API and the federated server-to-server API. Compatible with Mastodon, Pleroma and microblog.pub"
Pubstrate - experimental AP implementation written in GNU Guile (no docs?)
Proposed Projects and implementations in development
A place to list rumoured projects for further research, apps whose developers have expressed interest in implementing AP, and vapourware projects that have been planned but no usable code has been written yet. Once it's clear that the project really exists, is definitely working on implementing AP, and code is actually being committed, it can be moved to the appropriate category above.
Indienet - homepage - projects goals seem similar to the federated homepages of the IndieWeb, but federated using AP.
✒Marmota - service like Spotify or any music streaming service, initial commit;
Minds - a centralized attempt at a FB replacement, incorporating short or long form posts, and live chat, as well as an internal token system (a bit like Reddit gold?)
Spritely - a new federated media-streaming server in Ratchet, planned by Chris Webber of MediaGoblin fame
Listed on this watchlist just in case anyone doesn't realize they're dead, and thinks we just didn't know about them. Also in case a project is orphaned, then reactivated by a new developer.
Acorde - federated social music platform. Almost a year has passed since the initial commit.
✖CloutStream - proposed as a federated replacement for LinkedIn. Original Mastodon.social account appears to have been removed (copy on Pinafore), all the links projects tools appear to be down.
✖Fontina - proposed as a photo-sharing social media network. GH repo has gone read-only, and now says "dead project".
✖Numa - built on Ethereum blockchain but intends to federate with AP. No commits since April 2018. No reply on issue about AP compatibility testing from May 8.
✖Osada - Osada is a full featured social network application running under the ActivityPub protocol. It also communicates with and inter-operates with servers on the Zot6 network. Abandoned by the developers in March 2019.