You personal XMPP server!
Register
Web client
Russian instructions
Clients for mobile or tablet:
Conversations
Android (
F-Droid
for free)
C0NNECT
fork Conversations for Android
Blabber
Android
Monal
iOS
Siskin
iOS (Чиж)
Clients for PC:
Gajim
GNU/Linux and Windows
DINO
GNU/Linux