@pixelfed And a few with the letter M!
Mastodon - https://joinmastodon.org
Microblog - https://microblog.pub
Misskey - https://misskey.io
Mobilizon - https://joinmobilizon.org (coming soon!)
MoodleNet - https://moodle.com/moodlenet (coming soon!)
Mcabber - terminal client for federated chat https://mcabber.com/
Mellium - Go library for federated chat https://mellium.im/
Monal - iOS app for federated chat https://monal.im/
Movim - web based client for federated communication https://movim.eu/
Four more: #matrix https://www.ag-software.net/matrix-vnext/ #metronome https://metronome.im/ #miranda https://www.miranda-ng.org/ #mongooseim https://www.erlang-solutions.com/products/mongooseim.html
Server run by the main developers of the project It is not focused on any particular niche interest - everyone is welcome as long as you follow our code of conduct!