Messenger --https://www.messenger.com/
Instantly reach the people in your life—for free. Messenger is just like texting, but you don't have to pay for every message (it works with your data plan).
Trillian --http://www.trillian.im/
Trillian, the free instant messenger for Windows, MacOS X, Android, iPhone, BlackBerry, Linux, and the Web. Supports Windows Live, Facebook, Twitter, Yahoo, MySpace, AIM, Email, Google Talk, ICQ, Jabber, IRC, Bonjour, and more!
WhatsApp --www.whatsapp.com
WhatsApp Messenger: More than 1 billion people in over 180 countries use WhatsApp to stay in touch with friends and family, anytime and anywhere. WhatsApp is free and offers simple, secure, reliable messaging and calling, available on phones all over the world.