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 intends to federate with AP. No commits since early 2017. Dead project?
* [FunkWhale](https://medium.com/we-distribute/funkwhale-an-open-source-grooveshark-alternative-begins-activitypub-implementation-cbc10a412b20) - music streaming
* [Quit.im](https://quit.im/) this is a photo-sharing web client for a GNU social server, rather than a completely separate app, so it will become an AP implementation when the AP plug-in for GS is done.
* [Calendar-social](https://gitea.polonkai.eu/gergely/calendar-social/issues/122) - no specific mention of ActivityPub on the repo but rumour on the fediverse is that it will be an AP app. I will post a question on their issue tracker.
* [NextCloud](https://help.nextcloud.com/t/activitypub-the-new-standard-for-decentralized-networks/26381) - seem to be only using AP for internal federation?
* [ForgeFed](https://github.com/forgefed/forgefed/) (formerly GitPub) - a set of extensions to AP for federation between code forges (Git hosting sites like GitLab, Gogs, Gitea etc)