Conversations is a Jabber/XMPP client for Android 4.0+ smartphones that has been optimized to provide a unique mobile experience.
This is a modern, secure XMPP client I use each day to stay in touch with friends. It supports OMEMO encryption, message archives, integrates with the system well and is quite nice and intuitive.
For a desktop client, see Dino.
For a list of other clients, see XMPP Clients.