Slack, Discord, Rocket.Chat, Mattermost, Matrix.org, Threema, Cisco Jabber, Zulip, Keybase, Conversations, Milo, Kandan, HeySpace, Hubgets, Alterdesk, Voiswave, FASTEE, Engagedots, MainFrame, Vistasuite, ScudCloud, Minty Chat, Mazey, Modular.im are some alternatives to Async.