With pricing:
www.pubnub.com
quickblox.com
https://www.twilio.com/sms/api
https://www.applozic.com
https://www.hyphenate.io
opensource:
https://www.igniterealtime.org/projects/smack/
http://buddycloud.com/cooking-with-buddycloud-chat-app
https://chatsecure.org/developers/
Useful opensource projects that can be helpful:
http://www.java2s.com/Open-Source/Android_Free_Code/Chat/mobile/index.htm
https://www.javacodegeeks.com/2010/09/xmpp-im-with-smack-for-java.html
https://www.bennadel.com/blog/2213-sending-client-to-client-realtime-messages-with-the-pubnub-javascript-library.htm
https://github.com/faheema/ChatOn
http://www.blikoon.com/tutorials/android-smack-xmpp-introductionbuilding-a-simple-client
http://stackoverflow.com/questions/8542565/how-to-make-a-basic-instant-messaging-program-in-pure-java
