The SocialWG has a list of projects they hope will implement ActivityPub and links to Issues where it's discussed. Also, there is an implementation report on ActivityPub.rocks. More projects using AP are profiled on 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:

Social network, Microblog Apps

• Indienet

• Kroeg

• pump.io

• microblog.pub - single-user microblog server

• microstatus - "Lightweight Mastodon- and GNU social-compatible ActivityPub and OStatus server implementation"

• Misskey

• Numa - built on Ethereum blockchain but federates with AP

• Rustodon

• Smilodon/ Pylodon - web front-end / back-end written in Python

Blog and Publishing Apps

• Dokie.li

• Known

• Plume

Media-hosting Apps

• FunkWhale

• MediaGoblin

• PixelFed

Events Apps

• GetTogether

File, Contacts, and Calendar Syncing Apps

• NextCloud

• ownCloud

Dev Tools

• BridgyFed

• Distbin

• GitLab (see also: gitlab-org/gitlab-ce#30991)

• GitPub - a set of extensions to AP for federation between code forges (Git hosting sites like GitLab, Gogs, Gitea etc)

• Places.pub - AP implementation testing tool?

• Pubstrate - experimental AP implementation written in GNU Guile

• tags.pub - AP implementation testing tool?