HeyWire!

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.

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)

Pinger lets you text and talk free with the people you care about most, anywhere on the planet. Tap a picture and you'll see all of your communications—texts, pics, calls, voicemails—all in one place.

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

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

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.

Chat with people you know, or find new friends.Main Features:Share messages, photos, videos, sound/voice recordings and location.Notification of messages when application is not running.Profile picture and about information.Create your own groups or join existing ones. Groups can be public or private.PIN code to lock application and protect your privacy.Set favorite contacts and groups.Manual and automatic deletion of messages.

XendApp provides a messaging gateway for reaching public or internal users through applications for a variety of devices. Messages can be sent directly from http://xendapp.com web site or through any of the existing web services that the web site have. Messages can be sent for free or at a very low cost.