Note: none of these apps ought to be added to the official fediverse.party site until they have been tested with at least one (ideally 2-3) of the AP apps already listed, and can be confirmed to federate successfully. Once it is on the official site, it can be removed from this list, keeping it a list of apps to be watched/ tested.
The [SocialWG has a list](https://www.w3.org/wiki/Socialwg/ActivityPub_network) of projects they hope will implement ActivityPub and links to Issues where it's discussed. Also, there is an [implementation report on ActivityPub.rocks](https://activitypub.rocks/implementation-report/). @Mayel from Social.coop is maintaining a [web spreadsheet of AP apps](https://ethercalc.org/fediverse-stacks) and their characteristics. More projects using AP are [profiled on We Distribute](https://medium.com/we-distribute) by Sean Tilley. From these and other sources (see links), it looks like all these apps are either considering, working on, or already rolling out AP support:
* [microblog.pub](https://github.com/tsileo/microblog.pub) - single-user microblog server
* [microstatus](https://github.com/Arkanosis/microstatus) - "Lightweight Mastodon- and GNU social-compatible ActivityPub and OStatus server implementation"
* [Numa](https://github.com/numaverse/numaverse-gateway/issues/3) - built on Ethereum blockchain but federates with AP
* [Quit.im](https://quit.im/) this may just be an photo-sharing web client for a GNU social server, rather than a completely separate app, but if so, it will potentially become an AP implementation when the AP plug-in for GS is done.
* [GitLab](https://gitlab.com/gitlab-org/gitlab-ce/issues/4013) (see also: https://gitlab.com/gitlab-org/gitlab-ce/issues/30991)
* [GitPub](https://github.com/git-federation/gitpub) - a set of extensions to AP for federation between code forges (Git hosting sites like GitLab, Gogs, Gitea etc)
* [Osada](https://macgirvin.com/wiki/mike/Osada/Home) - a bridge server for smoother inter-operation between Zot networks (Hubzilla) and AP networks
* [Places.pub](https://github.com/w3c/activitypub/issues/282) - AP implementation testing tool?
* [Pubstrate](https://gitlab.com/dustyweb/pubstrate) - experimental AP implementation written in GNU Guile
* [tags.pub](https://github.com/w3c/activitypub/issues/281) - AP implementation testing tool?