Telegram is a mobile messenger which lets you text your friends with
Share text, voice messages, photos, videos, GIFs and files for free. Signal uses your phone's data connection so you can avoid SMS and MMS fees.
LINE is a communication app, Line has variety of features such as Text, Video, Audio Calling, Groups, Send Photos, Find People Nearby
Wechat is a messaging app and very popular in china, it is available on all platforms
Viber is a communication application with features such as audio, video, text chat, groups etc.
Facebook Messenger lets you send messages to your friends connected on your facebook account, Make Free Video and Audio calls to your friends and family using FB Messenger