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.
https://hashtag-relay.dtp-mstdn.jp/
https://github.com/dtp-mstdn-jp/selective-relay

There are others who run pub-relay-prototype and others who run relays written in Node.js, which they developed themselves.
https://relay.mstdn-jp.site/ui/
https://github.com/gittrname/activitypub-relay

Also, today we launched the Fedibird Relay Service with pub-relay 2.0, which allows Mastodon, Pleroma and Misskey to participate.
https://relay.fedibird.com/

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.
https://github.com/noellabo/pub-relay