Conversations (Jabber / XMPP) play online

A free and open source Jabber/XMPP client for Android. Easy to use, reliable, battery friendly. With built-in support for images, group chats and e2e encryption.

Design principles
Be as beautiful and easy to use as possible without sacrificing security or privacy
Rely on existing, well established protocols
Do not require a Google Account or specifically Google Cloud Messaging (GCM)
Require as little permissions as possible

Features
End-to-end encryption with either OMEMO or OpenPGP
Encrypted audio and video calls (DLTS-SRTP)
Sending and receiving images
Intuitive UI that follows Android Design guidelines
Pictures / Avatars for your Contacts
Syncs with desktop client
Conferences (with support for bookmarks)
Address book integration
Multiple Accounts / unified inbox
Very low impact on battery life

XMPP Features
Conversations works with every XMPP server out there. However XMPP is an extensible protocol. These extensions are standardized as well in so called XEPs. Conversations supports a couple of those to make the overall user experience better. There is a chance that your current XMPP server does not support these extensions. Therefore to get the most out of Conversations you should consider either switching to an
XMPP server that does or - even better - run your own XMPP server for you and your friends.
These XEPs are - as of now:

XEP-0065: SOCKS5 Bytestreams (or mod_proxy65). Will be used to transfer files if both parties are behind a firewall (NAT).
XEP-0163: Personal Eventing Protocol for avatars
XEP-0191: Blocking command lets you blacklist spammers or block contacts without removing them from your roster.
XEP-0198: Stream Management allows XMPP to survive small network outages and changes of the underlying TCP connection.
XEP-0280: Message Carbons which automatically syncs the messages you send to your desktop client and thus allows you to switch seamlessly from your mobile client to your desktop client and back within one conversation.
XEP-0237: Roster Versioning mainly to save bandwidth on poor mobile connections
XEP-0313: Message Archive Management synchronize message history with the server. Catch up with messages that were sent while Conversations was offline.
XEP-0352: Client State Indication lets the server know whether or not Conversations is in the background. Allows the server to save bandwidth by withholding unimportant packages.
XEP-0363: HTTP File Upload allows you to share files in conferences and with offline contacts. Requires an additional component on your server.