XMPP

Linux Desktop

Dino is a modern open-source chat client for the desktop.

iOS / iDevice

ChatSecure is a free and open source encrypted chat client for iOS. Source available on github.

• On MacOS systems: Beagle
• For iOS: Siskin IM or Snikket

XMPP cons / downsides

• Admin-in-the-middle - XMPP clients can’t distinguish between legitimate and injected messages, even if OMEMO is enabled. The XMPP client just displays injected messages as an unencrypted message from the sender.

If you are implementing an XMPP based system pay attention to this and this