GO SMS Pro!

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.

Remember when text messaging was like magic?

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™.

Handcent SMS is a free enhanced SMS and MMS application for Android.

You love your Android phone as much as we do? You SMS all the time? Then chompSMS is for you: it's so convenient, and has a great user interface with chat style text bubbles. For those paying high SMS rates (international or no plan) there's also the option to send much cheaper SMS messages.

Cherry SMS allows to send SMS messages worldwide at very attractive rates. It features no ads, no base fee, no set-up fee and the credits purchased are available for an unlimited period.

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.

Textra is a seriously beautiful, feature rich SMS and MMS replacement app.

If you’ve ever wanted to send more than just an emoji or gif while chatting to express exactly how you feel, Vidii, lets you do it with short video clips from movies, tv, and the internet. We let you say more than just typed words.

YAATA is influenced by the true spirit of Android. It is a SMS/MMS app that has so much to offer. You can personalize and configure this app to your definition of the perfect app. YAATA SMS is very powerful, reliable and the speed of the app is on another level. Even with all the exciting features this app has to offer the app size is under 3.9MB. This SMS app is highly user friendly and fulfils the wants and needs of the ultimate geek to a basic beginner user.

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.