Which Apps Are Similar to WhatsApp Messenger?

Viber
- Kik
- Textme
- Heywire 
- Talk.to
- Hike 
- Samsung's Chat On
- iMessage
- BBM (only on BlackBerry)
- GroupMe
- WeChat
Line Messenger: A cross platform messenger supported across Mobile (Android, iOS, Windows Phone) and PC (windows, mac)