#XMPP (aka
#Jabber) is an easy, widely used free federated chat system.
There are thousands of instances to sign up at, such as
https://chinwag.im and
https://www.xmpp.jpThere are also *lots* of apps.
Maybe the most important thing is that an app supports OMEMO privacy:
https://omemo.topSome recommended OMEMO-compatible apps:
Conversations (Android)
https://conversations.imGajim (Win/Linux)
https://gajim.orgMonal (iOS/Mac)
https://monal.im#Alternatives