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.
✒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?)
Pleroma relay (listed in the-federation.info and fediverse.network as ActivityRelay) - "Generic LitePub relay (works with all LitePub consumers and Mastodon)"
Proposed Projects amd 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.