mjoy!

WhatsApp Messenger is a cross-platform mobile messaging app which allows you to exchange messages without having to pay for SMS. WhatsApp Messenger is available for iPhone, BlackBerry, Android, Windows Phone and Nokia and yes, those phones can all message each other! Because WhatsApp Messenger uses the same internet data plan that you use for email and web browsing, there is no cost to message and stay in touch with your friends.

With Viber, everyone in the world can connect. Freely. More than 460 million Viber users text, make HD-quality phone and video calls, and send photo and video messages worldwide over WiFi or 3G - for free. Viber Out can be used to make calls to non-Viber landlines and mobile numbers at low rates. Viber is available for many smartphones and platforms.

Zulip is a group chat app.

KakaoTalk is a phone number based messenger service that allows users anywhere in the world to chat one-to-one or in a group for free. (no SMS service)

Group messaging platform owned by Microsoft. It's absolutely free, whether you're talking to a group of friends, or texting with one person. Best of all, it works on nearly every phone, via push or SMS. With GroupMe, it's easy to reach anyone, anytime, anywhere.

Dasher lets you do more than just text message. It's messaging with benefits:

SeaFour is the SeaFour.club community's chat engine, powered by Node.js and Socket.io.It's a socket webchat supporting aesthetic username customization and stylized messages using a markup language similar to 'LISP', and supports a multitude of alternative clients. It uses a free-as-in-freedom license that enables users to modify the software to best fit their needs.

Sesame is a messaging app that gives you a self-aware digital assistant to protect messages on your behalf: your own Personal Cognitive Messenger™.

textPlus is the best way to send unlimited text messages from your android, iPhone, iPod, kindle, and windows devices with your textPlus phone number free and calling for just pennies.

Spotbros is an app where you can discover and share all types of content through public chat groups (Spots) or with your friends through private chats. Download our messaging app and get 1TB of free storage in the Spotbros cloud

WowApp helps you do good through the Power of Sharing. We share all revenues with you. WowApp Messenger is free and you can receive earnings without ever spending money.

Open source instant messaging server and clients. Backend in pure Go (license GPL 3.0), client-side binding in Java for Android and Javascript (license Apache 2.0), persistent storage RethinkDB. JSON over websocket (long polling is also available). Tinode is meant as a replacement for XMPP. Overall it's a lot like open source WhatsApp or Telegram.