The ActivityPub module for Drupal implements the W3C ActivityPub protocol for websites based on this popular website management system. With the module, a website becomes an actual ActivityPub server. People are able to follow content from your site on Mastodon and other federated platforms that support ActivityPub. Responses are possible too (Reply, Like, Announce) – with more to come.
If you need a client, you can install the Drupal Personal Reader project, which allows you to view and interact with followers from your own Drupal instance. Soon you will also be able to use any (ActivityPub/Mastodon-compatible) client to connect with the in-built API.