Also, today we launched the Fedibird Relay Service with pub-relay 2.0, which allows Mastodon, Pleroma and Misskey to participate.
pub-relay 2.0 is a fork and maintenance of the Mastodon relays that I have stopped developing, with code added to accommodate Pleroma and Misskey.
The next most commonly used is the hashtag relay service, which relays only hashtagged posts. This is a fork of pub-relay (the sidekiq implementation) with added functionality, with code added to support Pleroma and Misskey. A distinctive feature is the ability to have accounts join the relay, with 167 servers and users participating on a per account basis, albeit privately.